Ageing

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Synonyms and keywords: Aging

## Overview

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The effects of aging on a human face An elderly woman

Ageing is any change in an organism over time. Ageing refers to a multidimensional process of physical, psychological, and social change. Some dimensions of aging grow and expand over time, while others decline. Reaction time, for example, may slow with age, while knowledge of world events and wisdom may expand. Research shows that even late in life potential exists for physical, mental, and social growth and development. Aging is an important part of all human societies reflecting the biological changes that occur, but also reflecting cultural and societal conventions. Age is usually measured in full years — and months for young children. A person's birthday is often an important event.

The term "aging" is somewhat ambiguous. Distinctions may be made between "universal aging" (age changes that all people share) and "probabilistic aging" (age changes that may happen to some, but not all people as they grow older, such as the onset of Type Two diabetes). Chronological aging, referring to how old a person is, is arguably the most straightforward definition of aging and may be distinguished from "social aging" (society's expectations of how people should act as they grow older) and "biological aging" (an organism's physical state as it ages). There is also a distinction between "proximal aging" (age-based effects that come about because of factors in the recent past) and "distal aging" (age-based differences that can be traced back to a cause early in person's life, such as childhood poliomyelitis).[1]

Differences are sometimes made between populations of children;divisions are sometimes made between the young old (65-74), the middle old (75-84) and the oldest old (those aged 85 and above). However, problematic in this is that chronological age does not correlate perfectly with functional age, i.e. two people may be of the same age, but differ in their mental and physical capacities.

Population aging is the increase in the number and proportion of older people in society. Population aging has three possible causes: migration, longer life expectancy (decreased death rate), and decreased birth rate. Aging has a significant impact on society. Young people tend to commit most crimes, they are more likely to push for political and social change, to develop and adopt new technologies, and to need education. Older people have different requirements from society and government as opposed to young people, and frequently differing values as well. Older people are also far more likely to vote, and in many countries the young are forbidden from voting. Thus, the aged have comparatively more political influence.

## Senescence

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Main article: Senescence

In biology, senescence is the state or process of aging. Cellular senescence is a phenomenon where isolated cells demonstrate a limited ability to divide in culture (the Hayflick Limit, discovered by Leonard Hayflick in 1965), while Organismal senescence is the aging of organisms.

After a period of near perfect renewal (in humans, between 20 and 35 years of age), organismal senescence is characterized by the declining ability to respond to stress, increasing homeostatic imbalance and increased risk of disease. This irreversible series of changes inevitably ends in death.

Some researchers (specifically biogerontologists) are treating aging as a disease. As genes that have an effect on aging are discovered, aging is increasingly being regarded in a similar fashion to other genetic conditions, potentially "treatable."

Indeed, aging is not an unavoidable property of life. Instead, it is the result of a genetic program. Numerous species show very low signs of aging ("negligible senescence'), the best known being trees like the bristlecone pine, fish like the sturgeon and the rockfish, invertebrates like the quahog or sea anemone [2].

In humans and other animals, cellular senescence has been attributed to the shortening of telomeres with each cell cycle; when telomeres become too short, the cells die. The length of telomeres is therefore the "molecular clock," predicted by Hayflick. Telomere length is maintained in immortal cells (e.g. germ cells and keratinocyte stem cells, but not other skin cell types) by the enzyme telomerase. In the laboratory, mortal cell lines can be immortalized by the activation of their telomerase gene, present in all cells but active in few cell types. Cancerous cells must become immortal to multiply without limit. This important step towards carcinogenesis implies, in 85% of cancers, the reactivation of their telomerase gene by mutation. Since this mutation is rare, the telomere "clock" can be seen as a protective mechanism against cancer [3].

Other genes are known to affect the aging process, the sirtuin family of genes have been shown to have a significant effect on the lifespan of yeast and nematodes. Over-expression of the RAS2 gene increases lifespan in yeast substantially.

In addition to genetic ties to lifespan, diet has been shown to substantially affect lifespan in many animals. Specifically, caloric restriction (that is, restricting calories to 30-50% less than an ad libitum animal would consume, while still maintaining proper nutrient intake), has been shown to increase lifespan in mice up to 50%. Caloric restriction works on many other species beyond mice (including species as diverse as yeast and Drosophila), and appears (though the data is not conclusive) to increase lifespan in primates according to a study done on Rhesus monkeys at the National Institute of Health (US). Since, at the molecular level, age is counted not as time but as the number of cell doublings, this effect of calorie reduction could be mediated by the slowing of cellular growth and, therefore, the lengthening of the time between cell divisions.

Drug companies are currently searching for ways to mimic the lifespan-extending effects of caloric restriction without having to severely reduce food consumption.

## Dividing the Lifespan

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A human life is often divided into various ages. Historically, the lifespan of man was divided into seven ages; because biological changes are slow moving and vary from person to person, arbitrary dates are usually set to mark periods of human life. In some cultures the divisions given below are quite varied.

In the USA, adulthood legally begins at the age of eighteen or nineteen, while old age is considered to begin at the age of legal retirement (approximately 65).

* Pre-conception: ovum, spermatozoon, possible pre-existence
* Conception: fertilization
* Pre-birth: conception to birth
* Infancy: Birth to 2
* Childhood: 2 to 11
* Adolescence: 12 to 19
* Early adulthood: 20 to 35
* Middle adulthood: 35 to 54
* Late adulthood: 55+
* Death
* Post-Death: Decomposition

Ages can also be divided by decade:

* Denarian: someone between 10 and 19 years of age
* Vicenarian: someone between 20 and 29 years of age
* Tricenarian: someone between 30 and 39 years of age
* Quadragenarian: someone between 40 and 49 years of age
* Quinquagenarian: someone between 50 and 59 years of age
* Sexagenarian: someone between 60 and 69 years of age
* Septuagenarian: someone between 70 and 79 years of age
* Octogenarian: someone between 80 and 89 years of age
* Nonagenarian: someone between 90 and 99 years of age
* Centenarian: someone over 100 years of age
* Supercentenarian: someone over 110 years of age

## Cultural Variations

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In some cultures (for example Serbian and Russian) there are two ways to express age: by counting years with or without including current year. For example, it could be said about the same person that he is twenty years old or that he is in twenty-first year of his life. In Russian the former expression is generally used, the latter one has restricted usage: it is used for age of a deceased person in obituaries and for age of a child when it is desired to show him/her older than he/she is. (It seems that a boy in his 4th year is older than one who is 3 years old.)

Considerable numbers of cultures have less of a problem with age compared with what has been described above, and it is seen as an important status to reach stages in life, rather than defined numerical ages. Advanced age is given more respect and status.

East Asian age reckoning is different from that found in Western culture. Traditional Chinese culture uses a different aging method, called Xusui (虛歲) with respect to common aging which is called Zhousui (周歲). In the Xusui method, people are born at age 1, not age 0.

## Society

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### Legal

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There are variations in many countries as to what age a person legally becomes an adult.

Most legal systems define a specific age for when an individual is allowed or obliged to do something. These ages include voting age, drinking age, age of consent, age of majority, age of criminal responsibility, marriageable age, age where one can hold public office, and mandatory retirement age. Admission to a movie for instance, may depend on age according to a motion picture rating system. A bus fare might be discounted for the young or old.

Similarly in many countries in jurisprudence, the defense of infancy is a form of defense by which a defendant argues that, at the time a law was broken, they were not liable for their actions, and thus should not be held liable for a crime. Many courts recognize that defendants who are considered to be juveniles may avoid criminal prosecution on account of their age.

### Economics and Marketing

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The economics of aging are also of great import. Children and teenagers have little money of their own, but most of it is available for buying consumer goods. They also have considerable impact on how their parents spend money.

Young adults are an even more valuable cohort. They often have jobs with few responsibilities such as a mortgage or children. They do not yet have set buying habits and are more open to new products.

The young are thus the central target of marketers.[4] Television is programmed to attract the range of 15 to 35 year olds. Movies are also built around appealing to the young.

### Health Care Demand

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Many societies in the rich world, e.g. Western Europe and Japan, have aging populations. While the effects on society are complex, there is a concern about the impact on health care demand. The large number of suggestions in the literature for specific interventions to cope with the expected increase in demand for long-term care in aging societies can be organized under four headings: improve system performance; redesign service delivery; support informal caregivers; and shift demographic parameters.[5]

However, the annual growth in national health spending is not mainly due to increasing demand from aging populations, but rather has been driven by rising incomes, costly new medical technology, a shortage of health care workers and informational asymmetries between providers and patients.[6]

Even so, it has been estimated that population aging only explains 0.2 percentage points of the annual growth rate in medical spending of 4.3 percent since 1970. In addition, certain reforms to Medicare decreased elderly spending on home health care by 12.5 percent per year between 1996 and 2000. [7] This would suggest that the impact of aging populations on health care costs is not inevitable.

### Impact on Prisons

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As of July 2007, medical costs for a typical inmate might run an agency around $33 per day, while costs for an aging inmate could run upwards of $100. Most DOCs report spending more than 10 percent of the annual budget on elderly care. That is expected to rise over the next 10-20 years. Some states have talked about releasing aging inmates early. [8]

## Cognitive Effects

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Steady decline in many cognitive processes are seen across the lifespan, starting in one's thirties. Research has focused in particular on memory and aging, and has found decline in many types of memory with aging, but not in semantic memory or general knowledge such as vocabulary definitions, which typically increases or remains steady. Early studies on changes in cognition with age generally found declines in intelligence in the elderly, but studies were cross-sectional rather than longitudinal and thus results may be an artefact of cohort rather than a true example of decline. Intelligence may decline with age, though the rate may vary depending on the type, and may in fact remain steady throughout most of the lifespan, dropping suddenly only as people near the end of their lives. Individual variations in rate of cognitive decline may therefore be explained in terms of people having different lengths of life.[1]

## Coping and Well-being

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Psychologists have examined coping skills in the elderly. Various factors, such as social support, religion and spirituality, active engagement with life and having an internal locus of control have been proposed as being beneficial in helping people to cope with stressful life events in later life.[9][10][11] Social support and personal control are possibly the two most important factors that predict well-being, morbidity and mortality in adults.[12] Other factors that may link to well-being and quality of life in the elderly include social relationships (possibly relationships with pets as well as humans), and health.[13]

Individuals in different wings in the same retirement home have demonstrated a lower risk of mortality and higher alertness and self-rated health in the wing where residents had greater control over their environment,[14][15] though personal control may have less impact on specific measures of health.[11] Social control, perceptions of how much influence one has over one's social relationships, shows support as a moderator variable for the relationship between social support and perceived health in the elderly, and may positively influence coping in the elderly.[16]

### Religion

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Religion has been an important factor used by the elderly in coping with the demands of later life, and appears more often than other forms of coping later in life.[17] Religious commitment may also be associated with reduced mortality, though religiosity is a multidimensional variable; while participation in religious activities in the sense of participation in formal and organized rituals may decline, it may become a more informal, but still important aspect of life such as through personal or private prayer.[18]

### Self-rated Health

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Self-ratings of health, the beliefs in one's own health as excellent, fair or poor, has been correlated with well-being and mortality in the elderly; positive ratings are linked to high well-being and reduced mortality.[19][20] Various reasons have been proposed for this association; people who are objectively healthy may naturally rate their health better than that of their ill counterparts, though this link has been observed even in studies which have controlled for socioeconomic status, psychological functioning and health status.[21] This finding is generally stronger for men than women,[20] though the pattern between genders is not universal across all studies, and some results suggest sex-based differences only appear in certain age groups, for certain causes of mortality and within a specific sub-set of self-ratings of health.[21]

### Retirement

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Retirement, a common transition faced by the elderly, may have both positive and negative consequences.[22]

## Societal Impact

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Societal aging refers to the demographic aging of populations and societies.[23] Cultural differences in attitudes to aging have been studied.

### Emotional Improvement

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Given the physical and cognitive declines seen in aging, a surprising finding is that emotional experience improves with age. Older adults are better at regulating their emotions and experience negative affect less frequently than younger adults and show a positivity effect in their attention and memory. The emotional improvements show up in longitudinal studies as well as in cross-sectional studies, and so cannot be entirely due to only the happier individuals surviving.

## Terminology

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The concept of successful aging can be traced back to the 1950s, and popularised in the 1980s. Previous research into aging exaggerated the extent to which health disabilities, such as diabetes or osteoporosis, could be attributed exclusively to age, and research in gerontology exaggerated the homogeneity of samples of elderly people.[24][25]

Successful aging consists of three components:[26]

1. Low probability of disease or disability;
2. High cognitive and physical function capacity;
3. Active engagement with life.

A greater number of people self-report successful aging than those that strictly meet these criteria.[24]

Successful aging may be viewed an interdisciplinary concept, spanning both psychology and sociology, where it is seen as the transaction between society and individuals across the life span with specific focus on the later years of life.[27] The terms "healthy aging"[24] "optimal aging" have been proposed as alternatives to successful aging.

Six suggested dimensions of successful aging include:[11]

1. No physical disability over the age of 75 as rated by a physician;
2. Good subjective health assessment (i.e. good self-ratings of one's health);
3. Length of undisabled life;
4. Good mental health;
5. Objective social support;
6. Self-rated life satisfaction in eight domains, namely marriage, income-related work, children, friendship and social contacts, hobbies, community service activities, religion and recreation/sports

## Theories

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### Non-biological Theories

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Modernization Theory
This is the view that the status of the elderly has declined since industrialization and the spread of technology.

Cognitive Theory
A view of aging that emphasizes individual subjective perception, rather than actual objective change itself, as the factor that determines behavior associated with advanced age.

Demographic Transition Theory
The idea that population aging can be explained by a decline in both birth rates and death rates following industrialization.

Disuse Theory
The idea states that cognitive and physical skills will atrophy unless one continuously practices them.

Exchange Theory
The idea that interaction in social groups is based on the reciprocal balancing of rewards depending on actions performed

Political Economy Theory
A societal perspective on the aging process that explains that the status and resources of the elderly, as well as how people age, are shaped by each person's place in the social structure and the economic and political forces that impact their sociopolitical location.

Disengagement Theory
This is the idea that separation of older people from active roles in society is normal and appropriate, and benefits both society and older individuals. Disengagement theory, first proposed by Cumming and Henry, has received considerable attention in gerontology, but has been much criticised.[1] The original data on which Cumming and Henry based the theory were from a rather small sample of older adults in Kansas City, and from this select sample Cumming and Henry then took disengagement to be a universal theory.[28] There are research data suggesting that the elderly who do become detached from society as those were initially reclusive individuals, and such disengagement is not purely a response to aging.[1]

Activity Theory
In contrast to disengagement theory, this theory implies that the more active elderly people are, the more likely they are to be satisfied with life. The view that elderly adults should maintain well-being by keeping active has had a considerable history, and since 1972, this has become to be known as activity theory.[28] However, this theory may be just as inappropriate as disengagement for some people as the current paradigm on the psychology of aging is that both disengagement theory and activity theory may be optimal for certain people in old age, depending on both circumstances and personality traits of the individual concerned.[1] There are also data which query whether, as activity theory implies, greater social activity is linked with well-being in adulthood.[28]

* Selectivity Theory - mediates between Activity and Disengagement Theory, which suggests that it may benefit older people to become more active in some aspects of their lives, more disengaged in others.[28]

Continuity Theory
The view that in aging people are inclined to maintain, as much as they can, the same habits, personalities, and styles of life that they have developed in earlier years. Continuity theory is Atchley's theory that individuals, in later life, make adaptations to enable them to gain a sense of continuity between the past and the present, and the theory implies that this sense of continuity helps to contribute to well-being in later life.[13] Disengagement theory, activity theory and continuity theory are social theories about ageing, though all may be products of their era rather than a valid, universal theory.

### Biological Theories

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Telomere Theory
Telomeres (structures at the ends of chromosomes) have experimentally been shown to shorten with each successive cell division. Shortened telomeres activate a mechanism that prevents further cell multiplication. This may be an important mechanism of aging in tissues like bone marrow and the arterial lining where active cell division is necessary.
Reproductive-Cell Cycle Theory
The idea that aging is regulated by reproductive hormones that act in an antagonistic pleiotropic manner via cell cycle signaling, promoting growth and development early in life in order to achieve reproduction, but later in life, in a futile attempt to maintain reproduction, become dysregulated and drive senescence (dyosis).
Wear-and-Tear theory
The idea that changes associated with aging are the result of chance damage that accumulates over time.
Somatic Mutation Theory
The biological theory that aging results from damage to the genetic integrity of the body’s cells.
Error Accumulation Theory
The idea that aging results from chance events that gradually damage the genetic code.
Evolutionary Theories
See Theories of aging in senescence. These have by far the most theoretical and empirical support.
Accumulative-Waste Theory
The biological theory of aging that points to a buildup of cells of waste products that presumably interferes with metabolism.
Autoimmune Theory
The idea that aging results from an increase in autoantibodies that attack the body's tissues. A number of diseases associated with aging, such as atrophic gastritis and Hashimoto's thyroiditis, are probably autoimmune in this way.
Aging-Clock Theory
The theory that aging results from a preprogrammed sequence, as in a clock, built into the operation of the nervous or endocrine system of the body. In rapidly dividing cells the shortening of the telomeres would provide just such a clock.
Cross-Linkage Theory
The idea that aging results from accumulation of cross-linked compounds that interfere with normal cell function.
Free-Radical Theory
The idea that free radicals (unstable and highly reactive organic molecules, also named reactive oxygen species or oxidative stress) create damage that gives rise to symptoms we recognize as aging.
Mitohormesis
It has been known since the 1930s that restricting calories while maintaining adequate amounts of other nutrients prevents aging across a broad range of organism. Recently, Michael Ristow has shown that this delay of aging is due to increased formation of free radicals within the mitochondria causing a secondary induction of increased antioxidant defence capacity.[29]

Entropy Theory
It was demonstrated in a program of television in Portugal (RTP, Abciência, Elixir of immortality) that it is possible to stimulate the mechanisms of cellular repairing, reducing the entropy significantly, which is the only cause of the senescence. With a specific set of biomolecules, it is possible to immortalize, for example, yeast cells in fatal conditions of thermal and/or oxidative stress.

## Measure of Age

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The normal point of time from where to measure the age of a human being is from birth. Age in prenatal development is normally measured in gestational age, taking the last menstruation of the woman as a point of beginning. Alternatively,fertilization age, beginning from fertilization can be taken.

Age is often rounded downward to an integer, where the time of birth is taken to have been 0:00 (in other words, the number of days is first rounded upward, before rounding downward to whole years). Thus the age range 4-11 is until the 12th birthday.

## Related Chapters

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* Aging brain
* Biodemography
* Biological immortality
* Calorie restriction
* Gerontology
* Life expectancy
* List of life extension-related topics
* Longevity
* Maturity (psychological)
* Memory and aging
* Michael Ristow
* Mitohormesis
* Population aging
* Retirement
* Senescence
* The Grim Reaper Gene
* Youth bulge

## References

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1. ↑ 1.0 1.1 1.2 1.3 1.4 Stuart-Hamilton, Ian. The Psychology of Ageing: An Introduction. London: Jessica Kingsley Publishers. ISBN 1-84310-426-1.
2. ↑ Physiological Basis of Aging and Geriatrics, By Paola S. Timiras, p.26, Published 2003 Informa Health Care, ISBN 0849309484
3. ↑ name="pmid10647931">Hanahan D, Weinberg RA (2000). "The hallmarks of cancer". Cell. 100 (1): 57–70. PMID 10647931.
4. ↑ Krulwich, Robert (2006). "Does Age Quash Our Spirit of Adventure?". All Things Considered. NPR. Retrieved 2006-08-22.
5. ↑ Saltman, R.B. (2006). "The Impact Of Aging On Long-term Care In Europe And Some Potential Policy Responses" (PDF). International Journal of Health Services. 36 (4): 719–746. Retrieved 2008-02-11. Unknown parameter `|coauthors=` ignored (help)
6. ↑ Rheinhardt, Uwe E. (2003). "Does The Aging Of The Population Really Drive The Demand For Health Care?" (PDF). Health Affairs. 22 (6): 27–39. Retrieved 2008-04-17.
7. ↑ Meara, Ellen (2004). "Trends in medical spending on by age, 1963-2000" (PDF). Health Affairs. 23 (4): 176–183. Unknown parameter `|accesssdate=` ignored (`|access-date=` suggested) (help); Unknown parameter `|coauthors=` ignored (help)
8. ↑ http://www.corrections.com/news/article/16115 Growing burden for aging population
9. ↑ Schulz R, Heckhausen J (1996). "A life span model of successful aging". Am Psychol. 51 (7): 702–14. PMID 8694390.
10. ↑ Windsor TD, Anstey KJ, Butterworth P, Luszcz MA, Andrews GR (2007). "The role of perceived control in explaining depressive symptoms associated with driving cessation in a longitudinal study". Gerontologist. 47 (2): 215–23. PMID 17440126.CS1 maint: Multiple names: authors list (link)
11. ↑ 11.0 11.1 11.2 Diane F. Gilmer; Aldwin, Carolyn M. (2003). Health, illness, and optimal aging: biological and psychosocial perspectives. Thousand Oaks: Sage Publications. ISBN 0-7619-2259-8.CS1 maint: Multiple names: authors list (link)
12. ↑ Smith GC, Kohn SJ, Savage-Stevens SE, Finch JJ, Ingate R, Lim YO (2000). "The effects of interpersonal and personal agency on perceived control and psychological well-being in adulthood". Gerontologist. 40 (4): 458–68. PMID 10961035.CS1 maint: Multiple names: authors list (link)
13. ↑ 13.0 13.1 Bowling, Ann (2005). Ageing well: quality of life in old age. [Milton Keynes]: Open University Press. ISBN 0335215092.
14. ↑ Langer EJ, Rodin J (1976). "The effects of choice and enhanced personal responsibility for the aged: a field experiment in an institutional setting". J Pers Soc Psychol. 34 (2): 191–8. PMID 1011073.
15. ↑ Rodin J, Langer EJ (1977). "Long-term effects of a control-relevant intervention with the institutionalized aged". J Pers Soc Psychol. 35 (12): 897–902. PMID 592095.
16. ↑ Bisconti, T.L. (1999). "Perceived social control as a mediator of the relationships among social support, psychological well-being, and perceived health". The Gerontologist. 39 (1): 94–103. Retrieved 2008-02-11.
17. ↑ McFadden, S (2005), Gerontology and the Psychology of Religion; inPark, Crystal L.; Raymond F. Paloutzian. Handbook of the Psychology of Religion and Spirituality. New York: The Guilford Press. ISBN 1-57230-922-9.CS1 maint: Multiple names: authors list (link)
18. ↑ Mindel CH, Vaughan CE (1978). "A multidimensional approach to religiosity and disengagement". J Gerontol. 33 (1): 103–8. PMID 618958.
19. ↑ Idler, E.L. (2003). "Discussion: Gender Differences in Self-Rated Health, in Mortality, and in the Relationship Between the Two". The Gerontologist. 43 (3): 372–375. Retrieved 2008-02-11.
20. ↑ 20.0 20.1 Deeg, D.J.H. (2003). "Self-Rated Health, Gender, and Mortality in Older Persons: Introduction to a Special Section". The Gerontologist. 43 (3): 369–371. Retrieved 2008-02-11. Unknown parameter `|coauthors=` ignored (help)
21. ↑ 21.0 21.1 Benyamini, Y. (2003). "Gender Differences in the Self-Rated Health-Mortality Association: Is It Poor Self-Rated Health That Predicts Mortality or Excellent Self-Rated Health That Predicts Survival?". The Gerontologist. 43 (3): 396–405. Retrieved 2008-02-11. Unknown parameter `|coauthors=` ignored (help)
22. ↑ Panek, Paul E.; Hayslip, Bert (1989). Adult development and aging. San Francisco: Harper & Row. ISBN 0060450126.CS1 maint: Multiple names: authors list (link)
23. ↑ Sarah Harper, 2006, Aging Societies: Myths, Challenges and Opportunities.
24. ↑ 24.0 24.1 24.2 Strawbridge WJ, Wallhagen MI, Cohen RD (2002). "Successful aging and well-being: self-rated compared with Rowe and Kahn". Gerontologist. 42 (6): 727–33. PMID 12451153.CS1 maint: Multiple names: authors list (link)
25. ↑ Rowe JW, Kahn RL (1987). "Human aging: usual and successful". Science. 237 (4811): 143–9. PMID 3299702.
26. ↑ Rowe JW, Kahn RL (1997). "Successful aging". Gerontologist. 37 (4): 433–40. PMID 9279031.
27. ↑ Fentleman, DL (1990), Successful aging in a postretirement society Unknown parameter `|coauthors=` ignored (help); in Baltes, Margret M.; Baltes, Paul B. (1990). Successful aging: perspectives from the behavioral sciences. Cambridge, UK: Cambridge University Press. ISBN 052143582X.CS1 maint: Multiple names: authors list (link)
28. ↑ 28.0 28.1 28.2 28.3 Willis, Sherry L. (1996). Adult development and aging. New York, NY: HarperCollins College Publishers. ISBN 0673994023.
29. ↑ Schulz TJ, Zarse K, Voigt A, Urban N, Birringer M, Ristow M (2007). "Glucose restriction extends Caenorhabditis elegans life span by inducing mitochondrial respiration and increasing oxidative stress". Cell Metab. 6 (4): 280–93. doi:10.1016/j.cmet.2007.08.011. PMID 17908557.CS1 maint: Multiple names: authors list (link)

## External Links

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* The University of Southern California Davis School of Geronbtology
* The arrow of time \- a photo-essay of a family aging, with yearly portraits over 30 years
* National Institute on Aging (U.S.)

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