Aging is the accumulation of changes in a living organism over time. Most organisms have a predictable pattern of development.

Jerry and Saba at 32

Since grade school, Jerry was always the best athlete among his peers. Even in his mid-20s, when most of his friends gave up the weekly softball game, he was still the best pitcher, best hitter, and best runner. Now at the age of 32, he is losing his love of the game. Not only are the younger players better athletes, but he is having difficulty in coming close to his best game. In previous years, he was always getting better at the game, but now he realizes that he cannot take age-related improvement for granted. He recognizes that his wife, Saba, is beginning to gain weight even as she makes efforts to retain her figure. But she looks much younger than her younger sister, who smoked for many years and now has dry skin and the beginnings of wrinkles. Saba's work requires her to update her skills through yearly training exams. During these trainings, she realizes that her memory skills are not as strong as they used to be, yet she still is able to do very well on the training assessment. Jerry and Saba enjoy the company of their friends, but now their reciprocal relationship as a married couple has replaced outside friendships and the importance those once held in their lives. On the threshold of middle age, Jerry and Saba are experiencing the changes that come with more advanced aging.

Humans accumulate change in three different domains: biological, psychological, and social. As people grow older, their bodies first grow stronger and then begin to deteriorate. Their psychological characteristics also change over time as they exhibit shifting personality traits and use different intellectual abilities. Aging in the social domain is marked by the changing expectations that others have of people as they age.

Aging and Child Development

Biologically, aging begins with prenatal * development. The fetal period is the time when the developing fetus * gradually becomes a viable human being who will be able to survive outside of the uterus * . The accumulated successes of the prenatal period make it likely that a healthy infant will be delivered and will grow in strength, mass, and physical ability for the next 25 to 30 years.

The faces of aging, from child to senior.

The faces of aging, from child to senior.
Frank Schwere/Getty Images.

Childhood as a period of aging extends from age two until puberty * . During this time, the child grows continuously. Before the growth spurt associated with puberty, an average child will achieve about 85 percent of adult height. During childhood, the child becomes more adept at gross motor skills (such as those used in sports) and fine motor skills (such as those used in drawing or writing).

Psychologically, childhood is the time during which a person begins to develop self-reliance. The child also recognizes similarities and differences between self and others and begins to develop a self-concept. New mental abilities enable the child to learn what is taught at school and in other social settings. What is taught and what is seen as important to the child's development and education are culturally and environmentally determined.

In the social domain, children begin spending time outside the home and therefore have to learn to make friends. As they grow, their concept of friendship evolves. Preschool children accept any playmate as a friend, but as children grow older, they become selective. They also have to learn rules of society. As children generally like to experiment with new surroundings and new opportunities, they inevitably come to realize that their actions and choices have consequences.

Adolescence is a time of rapid change in all three domains. Physically, the adolescent is changing from a child to an adult. The onset of puberty, accompanied by a growth spurt (coming a little earlier for girls than for boys), is the time at which the physical body first becomes capable of sexual reproduction. In addition to changes in the reproductive organs and reproductive functions, the secondary sexual characteristics (traits that distinguish the two sexes) begin to appear.

In the psychological domain, there is exploration of all kinds, as the adolescent works to develop a sense of identity. It is important that adolescents have positive exploratory experiences, and the hoped-for outcome is that they have acquired a vocational goal, individual values, and a mature personality. Adolescents are aided in this exploration by their more advanced and more sophisticated cognitive abilities. In comparison to children, adolescents are more capable of abstract thinking. For the first time in their lives, they think about the future in a serious way. Young people at the threshold of adult life are likely to be idealistic.

Aging after Young Adulthood

After adolescence, the physical body continues to grow stronger, reaching a peak or plateau between 25 and 30 years of age. Starting in one's 30s, the slow degeneration of the physical body that is part of the aging process begins. While adults gradually lose physical abilities, overall mental abilities tend to increase through middle age. Research shows that learning continues throughout the lifespan. The social effects of aging are often seen in how adults draw satisfaction from their unique experiences rather than from novelty and excitement, which the young often crave and regard as essential.


Each species has a predictable life expectancy, and there is a definite relationship between genes * and lifespan. Researchers at the National Institute of Aging identified individual fruit flies with a “longevity gene,” and through selective breeding doubled the lifespan of the fruit fly in their laboratory. This experiment led many to hope that a similar gene that exerts control over the rate of the aging process may be found in humans to help regulate the rate of the aging process.

Some gerontologists (medical doctors researching the causes and effects of aging) believe that aging should be regarded as a treatable condition. While it is impossible to prevent aging altogether, these professionals work to delay the effects of aging for as long as possible. The goal is to increase the productive lifespan of humans and not to merely lengthen their number of years.

Many people like to think they can minimize the decline in physical ability by maintaining a positive mental outlook. However, professional athletes, for example, cannot deny that in middle age there is a sharp decline in physical ability, agility, and endurance when compared to their earlier peak performances. At the same time, there are middle-aged athletes who perform better than younger ones who have not made a habit of daily exercise. Exercise and health patterns throughout life play an important role in the maintenance of physical vigor in the years after the late 20s. After the age of 35, female reproduction capability decreases more sharply, but men of comparable age do not have an equivalent decline in their likelihood of becoming fathers. However, there is a distinct decline in the number and quality of their sperm cells. Around the age of 50, women enter menopause, which marks the end of their childbearing years.

While middle-aged people who maintain their health may reduce the impact of age-related physical degeneration, the decline in sensory acuity is unavoidable. Vision decline is slow between the ages of 30 and 70, but after 70 it is more rapid. There is a similar trend for the sense of smell, where there is virtually no decline in middle age but a sharp decline in the late 60s. Hearing has a steady decline every 10 years from the age of 30 to 60, when the decline becomes steeper. The body begins responding less effectively to stress. The risk of many diseases starts to increase.

Alzheimer's Disease

Alzheimer's disease is a progressive degenerative (progressively worsening or becoming more impaired) condition characterized by decreasing memory and brain functions and the gradual loss of identity and life memories. The cause of Alzheimer's disease is not fully understood; however, there seems to be a strong genetic component in the early-onset form of the illness, but a smaller genetic influence in the normal or late-onset form. Some researchers think that those who are at risk of Alzheimer's disease (based on family histories) may be able to delay the onset of the disease through mental exercises.

In the elderly, there is a higher incidence of dementia * . In many elderly people, dementia is attributable to brain injury. The dementia may represent the accumulated effects of alcoholism, the presence of ministrokes, or a brain tumor * . A common form of dementia is Alzheimer's (ALTS-hy-merz) disease.

From middle age onward, human neural processing starts slowing down, particularly reaction time, perceptual accuracy, and ability to perform memory tasks. Brain size changes with age, so that an 80-year-old's brain is on average about 10 percent smaller than it was at the age of 20. The areas of the brain most affected by this loss are the ones associated with memory. However, it appears that being mentally active can counter this trend, as it is likely that mental activity encourages the growth of new neurons and synaptic connections. There is evidence that maintaining a physical exercise regimen throughout one's life, just as it contributes to one's physical agility, contributes to one's mental agility later in life.

One popular assumption in the United States is that growing older is negative, and that it is associated with increased difficulties and decreased intelligence. Conclusive evidence for reduced overall intelligence as part of the aging process has not been established. When a group of people take an intelligence test at the same time, the younger test-takers tend to do better than the older ones, which suggests an inverse relationship between age and test-taking ability. This finding would seem to suggest that as people age, they tend to lose intellectual capabilities. However, in longitudinal studies, in which the same people are given intelligence tests many times over a long period of time, there is no decrease in intelligence. In fact, there appears to be an increase in intellectual functioning in middle age.

With regard to the social aspect of midlife aging, one misconception needs dispelling: the midlife crisis. Many people expect that middle age will be a period of larger problems and less satisfaction. However, there is no scientific justification for such an expectation. On average, there is no increased emotional instability in midlife. Also, there is no decrease in the measure of life satisfaction associated with middle age and old age.

According to the psychologist Erik Erikson (1902–1994), much psychosocial development continues in adulthood and late life. Typically, young adults are focused on finding a mate; middle-aged adults, on leaving something behind for future generations; and people in late life, on seeking validation that their lives have been meaningful. Those who marry and have children eventually experience an empty nest, when the children grow up and begin separate lives of their own. For many people, this transition is happy, as a new milestone is attained.

Death is the ultimate effect of aging. People tend to accept death in different ways based on their personal, religious, and cultural views. Grief is an individual experience; there is no model for grief that all individuals follow.

Average Life Expectancy in Years for Men and Women in the United States, 1900–2010 Hyattsville, MD: National Center for Health Statistics, 2015.

Average Life Expectancy in Years for Men and Women in the United States, 1900–2010 Hyattsville, MD: National Center for Health Statistics, 2015.
SOURCE: Arias, E.“United States Life Tables, 2011.” National Vital Statistics Reports; vol 64 no 11.

Aging Populations in Modern Times

While more people enjoy longer, more productive lives in the 21st century, there are problems associated with an aging population. Due to better living conditions, better medical treatments, and more nutritious eating habits in modern times than in previous ages, the elderly populations of many countries are increasing. At the same time, the birth rates for many of these countries are decreasing. For the first time in human history, in the United States and elsewhere the population of people over the age of 65 years is greater than the population of children under the age of five. It was anticipated that as middle-aged persons begin to retire there would not be enough younger workers to take their places. One benefit of better health among the elderly is that workers may postpone retirement for many years. However, the elderly will continue to need more health care, which may stress the health care system as the elderly live longer.

Another risk factor for early death is a sedentary lifestyle—a lifestyle that does not include physical exercise on most days of the week. According to the results of several studies published in 2015, sitting for long periods of time on most days is dangerous, even for people who exercise regularly. Prolonged sitting can lead to heart disease, diabetes, and a condition known as fatty liver. Exercise is also important to maintaining mobility in older adults. Participation in regular exercise has also been proven to reduce depression in both young people and the elderly.

In addition, emotions can affect how bodies age. For example, stress and anger trigger the release of hormones that, over time, can lead to heart disease and death. Stress may also play a role in the development of Alzheimer's disease and other forms of dementia. Research reported in late 2015 revealed that people in their 40s who held negative assumptions about aging and the elderly showed a greater loss in the volume of the portion of the brain whose loss is linked to the development of Alzheimer's when their brains were scanned 25 years later as compared to those who voiced a more positive outlook toward the aging process when they were in their 40s. It is also known that while high levels of stress over time can accelerate the aging process by increasing inflammation, which has been linked to the development of major diseases including cancer and heart disease, taking steps to reduce stress can have the opposite effect. For example, participating regularly in activities such as meditation has been shown to slow the biological signs of aging.

Finally, in 2016 a clinical trial, known as the Targeting Aging with Metformin (TAME) trial, began enrolling 3,000 people between the ages of 65 and 80 years who had been diagnosed with or who were at high risk for the development of cancer, heart disease, and/or dementia. Metformin is a drug which has been used to treat or even prevent diabetes in some people. Growing research related to metformin indicated the drug may have the potential for reducing risk for conditions other than diabetes that are associated with the aging process including cardiovascular disease, some types of cancer, and glaucoma, a condition of the eyes associated with aging. Some researchers think metformin may be instrumental in slowing down aspects of the aging process itself. One-half of the people enrolled in the clinical trial will be given metformin; all participants in the trial will be tracked for six years. Researchers said the goal of the trial is not to make people live longer but to increase their lifespan by living a healthier life. Another goal of the research is to gain insights into next-generation drugs that could slow down the aging process.

See also Alzheimer's Disease • Cancer: Overview • Deafness and Hearing Loss • Dementia • Diabetes • Heart Disease: Overview • Puberty and Sexual Development


Books and Articles

Bensadon, Benjamin. Psychology and Geriatrics: Integrated Care for an Aging Population. London: Academic Press, 2015.

Berger, Kathleen Stassen. The Developing Person through the Lifespan. 9th ed. New York: Worth, 2014.


Centers for Disease Control and Prevention. “Healthy Aging.” . (accessed March 17, 2016).


AARP (formerly American Association of Retired Persons). 601 E St. NW, Washington, DC 20049. Toll-free: 888-687-2277. Website: (accessed March 17, 2016).

National Institute on Aging. 31 Center Dr., MSC 2292, Building 31, Room 5C27, Bethesda, MD 20892. Telephone: 301-496-1752. Website: (accessed March 17, 2016).

* prenatal (pre-NAY-tal) means existing or occurring before birth, with reference to the fetus.

* fetus (FEE-tus) is the term for an unborn human after it is an embryo, from nine weeks after fertilization until childbirth.

* uterus (YOO-teh-rus) is the muscular, pear-shaped internal organ in a woman where a baby develops until birth.

* puberty (PU-ber-tee) is the period during which sexual maturity is attained.

* genes (JEENS) are chemical structures composed of deoxyribonucleic acid (DNA) that help determine a person's body structure and physical characteristics. Inherited from a person's parents, genes are contained in the chromosomes found in the body's cells.

* dementia (dih-MEN-sha) is a loss of mental abilities, including memory, understanding, and judgment.

* tumor (TOO-mor) is an abnormal growth of body tissue that has no known cause or physiologic purpose. A tumor may or may not be cancerous.

Disclaimer:   This information is not a tool for self-diagnosis or a substitute for professional care.

(MLA 8th Edition)