Aging is a gradual, continuous process of spontaneous change that begins at birth and continues throughout all stages of life. It involves maturation and development for children, adolescents, and young adults. Then, during middle and late age, many bodily functions decline. Thus, aging has positive and negative aspects.

People do not become old or elderly at any specific age. Traditionally, age 65 has been designated as the beginning of old age. But the reason was based in history, not biology. Age 65 was chosen as the age for retirement in Germany, the first nation to establish a retirement program, and it continues to be the retirement age for most people in developed societies, although this tradition is changing.

When a person becomes old can be answered in different ways:

• Chronologic age: Chronologic age is based solely on the passage of time. It is a person's age in years. Chronologic age has limited significance in terms of health. Nonetheless, the likelihood of developing a health problem increases as people age. Because chronologic age helps predict many health problems, it has some legal and financial uses.
• Biological age: Biological age refers to changes in the body that commonly occur as people age. Because these changes affect some people sooner than others, some people are biologically old at 40, and others are biologically young at 60 and even older.
• Psychologic age: Psychologic age is based on how people act and feel. For example, an 80-year-old who works, plans, looks forward to future events, and participates in many activities is considered psychologically young.

Normal Aging: People often wonder whether what they are experiencing as they age is normal or abnormal. Although people age somewhat differently, many changes occur in almost everyone. Some of these changes seem to result from internal processes, that is, from aging itself. Thus, such changes, although undesired, are considered normal. They are to be expected and are generally unavoidable. For example, as people age, the lens of the eye thickens, stiffens, and becomes less able to focus on close objects, such as reading materials (a disorder called presbyopia). This change occurs in virtually all older people, and no cause or explanation has been identified other than aging itself. Thus, presbyopia is considered normal aging. Other terms used to describe these changes are usual aging and senescence.

Did You Know... Average life expectancy has increased a lot, but maximum life span has increased little if at all.

Exactly what constitutes normal aging is not always clear. Changes that occur with normal aging make people more likely to develop certain disorders. However, people can sometimes take actions to compensate for these changes. For example, older people are more likely to lose teeth. But seeing a dentist regularly, eating fewer sweets, and brushing and flossing regularly may reduce the chances of tooth loss. Thus, tooth loss, although common with aging, may be an avoidable part of aging.

Also, functional decline that is part of aging sometimes seems similar to functional decline that is part of a disorder. For example, with advanced age, a decline in mental function is nearly universal and is considered normal aging. This decline includes increased difficulty learning new languages and increased forgetfulness. In contrast, the decline that occurs in dementia is much more severe. For example, people who are aging normally may misplace things or forget details, but people who have dementia may forget entire events. People with dementia also have difficulty doing normal daily tasks (such as driving, cooking, and handling finances) and understanding the environment, including knowing what year it is and where they are. Thus, dementia is considered a disorder, even though it is common in late life (see Delirium and Dementia). Certain kinds of dementia, such as Alzheimer's disease, differ from normal aging in other ways as well. For example, brain tissue (obtained during autopsy) in people with Alzheimer's disease looks different from that in older people without the disease. So the distinction between normal aging and dementia is clear.

Sometimes the distinction between functional decline that is part of aging and functional decline that is part of a disorder seems arbitrary. For example, as people age, blood sugar levels increase more after eating carbohydrates than they do in younger people. This increase is considered normal aging. However, if the increase exceeds a certain level, diabetes, a disorder, is diagnosed (see Diabetes Mellitus (DM): Diabetes Mellitus). In this case, the difference is one of degree only.

Healthy (Successful) Aging: Healthy aging refers to postponement of or reduction in the undesired effects of aging. The goals of healthy aging are maintaining physical and mental health, avoiding disorders, and remaining active and independent. For most people, maintaining general good health requires more effort as they age. Developing certain healthy habits—following a nutritious diet, exercising regularly, and staying mentally active—can help. The sooner a person develops them, the better. However, it is never too late to begin. In this way, people can have some control over what happens to them as they age.

Some evidence suggests that in the United States, healthy aging is on the rise:

• A decrease in the percentage of older people residing in nursing homes (even though the percentage of people over age 65 and of those over age 85 has increased in the general population)
• A decrease in the percentage of people aged 75 to 84 who report impairments
• A decrease in the percentage of people over age 65 with debilitating disorders

Studying Aging
Gerontology is the study of the aging process including physical, mental, and social changes. The information is used to develop strategies and programs for improving the lives of older people. Some gerontologists have a medical degree and are also geriatricians. Geriatrics is the branch of medicine that specializes in the care of older people, which often involves managing many disorders and problems at the same time. Geriatricians have studied the aging process so that they can better distinguish which changes result from aging itself and which indicate a disorder.

Life Expectancy

The average life expectancy of Americans has been increasing dramatically over the past century. A male child born in 1900 could expect to live only 46 years, and a female child, 48 years. Today, however, a male child can expect to live more than 73 years, and a female child, nearly 80 years. Although much of this gain can be attributed to the significant decrease in childhood mortality, life expectancy at every age beyond 40 has also increased dramatically. For example, a 70-year-old man can now expect to live beyond age 83, and a 70-year-old woman, beyond age 85.

Despite the increase in average life expectancy, the maximum life span—the oldest age to which people can live—has changed little since records have been kept. Despite the best genetic makeup and health care, no one seems to live much beyond 125 years, although some experts suggest that this number may be slowly increasing. Currently, a person has a 1 in 2 billion chance of living to the age of 120.

Several factors influence life expectancy:

• Heredity: Heredity influences whether a person will develop a disorder. For example, a person who inherits genes that increase the risk of developing high cholesterol levels is likely to have a shorter life. A person who inherits genes that protect against coronary artery disease and cancer is likely to have a longer life.
• Lifestyle: Avoiding smoking, not abusing drugs and alcohol, maintaining a healthy weight and diet, and exercising help people function well and avoid disorders.
• Exposure to toxins in the environment: Such exposure can shorten life expectancy even among people with the best genetic makeup.
• Health care: Preventing disorders or treating disorders after they are contracted, especially when the disorder can be cured (as with infections and sometimes cancer), helps increase life expectancy.

Last full review/revision August 2007 by Richard W. Besdine, MD