Advanced adult

Old age consists of ages nearing the average life span of human beings, and thus the end of the human life cycle. Euphemisms and terms for older people include advanced adult, elderly, and senior (chiefly US) or senior citizen and pensioner. Older people have limited regenerative abilities and are more prone to disease, syndromes, and sickness than other adults. For the biology of aging see senescence.

Old age cannot be defined exactly because it does not have the same meaning in all societies. In many parts of the world, people are considered old because of certain changes in their activities or social roles. For example, people may be considered old when they become grandparents or when they begin to do less or different work. In the United States, people are often considered old if they have lived a certain number of years. Many Americans think of 65 as the beginning of old age because United States workers become eligible to retire with full Social Security benefits at age 65. People in the 65-and-over age group are often called senior citizens. Starting in the year 2003, the age at which a person becomes eligible for full Social Security benefits will increase gradually until it reaches age 67 in 2027.

Worldwide, the number of people 65 or older is increasing faster than ever before. Most of this increase is occurring in developing countries. In the United States the percentage of people 65 or older increased from 4 percent in 1900 to about 13 percent in the late 1990s. In 1900, only about 3 million of the nation's people had reached 65. By 1998, the number of senior citizens had increased to about 34 million. Population experts estimate that more than 50 million Americans--about 17 percent of the population--will be 65 or older in 2020. The number of elderly people is growing around the world chiefly because more children reach adulthood. In most parts of the world, women live, on average, longer than men. In the United States in the late 1990s, life expectancy at birth was 80 years for women and 73 years for men. American women who were age 65 in the late 1990s could expect to live about 19 additional years. Men who were 65 could expect to live about 16 additional years.

In Western societies, adults are declared to be "old" when they reach the ages of 65-70, and secure their pension entitlement. Some governments offer old age pensions, and redeemable retirement savings plans.

Retirement is a typical lifestyle embraced by advanced adults, marking the end of a lifetime of work.

In the late 20th century and early 21st century, advances in nutrition and health care have extended the period of good health as well as extending the overall life expectancy.

Pensions provision relied upon life expectancy at retirement being short: This change has led to the pensions crisis.

Some believe there to be prejudice against older people in Western cultures, referred to as ageism.

The medical study of the aging process is gerontology, and the study of diseases that afflict the elderly is geriatrics.