Developmentalists distinguish between life span and life expectancy when they talk about biological aspects of aging. The term life span is used to describe the upper boundary of a species' life, the maximum number of years any individual can live. The maximum number of years human beings can live is about 120. Homo sapiens is believed to have one of the longest life spans of any species, if not the longest.

The term life expectancy is used to describe the number of years that will probably be lived by the average person born in a particular year. Improvements in medicine, nutrition, exercise, and lifestyle have increased our life expectancy an average of 30 additional years since 1900. The life expectancy of individuals born today in the United States is 77 years (80 for women, 73 for men). One in three women born today is expected to live to be 100 or more years of age. Worldwide, the population of individuals 65 years of age and older doubled from 1950 to 1990, and the fastest-growing segment of the population is those 85 years and older.

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Although life expectancy has increased dramatically, life span does not seem to have increased since the beginning of recorded history. Even if we are remarkably healthy throughout our adult lives, we begin to age at some point.

Many biological theories of aging have been proposed, but two that look within the body's cells for causes of aging merit special attention:

• Cellular clock theory. Leonard Hayflick (1977) suggests that cells can divide a maximum number of about 100 times and that, as we age, our cells become less capable of dividing. Hayflick found that cells extracted from adults in their fifties to seventies had divided fewer than 100 times; the total number of cell divisions was roughly related to the age of the individual. Hayflick places the upper limit on the human life span at about 120 years. In the last decade, scientists have tried to explain why cells lose their ability to divide. The answer may lie at the tips of chromosomes. Each time a cell divides, the telomeres that protect the ends of chromosomes become shorter and shorter. After about 100 replications, the telomeres are dramatically reduced, and the cell no longer can reproduce. One study linked age-related telomere erosion with an inability to recover from stress and an increase in cancer (Rudolf and others, 1999).

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• Free radical theory. Others speculate that people age because unstable oxygen molecules known as "free radicals" are produced in cells. These molecules ricochet within the cells, damaging DNA and other cellular structures (Knight, 2000). The damage done by free radicals may lead to a range of disorders, including cancer and arthritis.

The aging process produces many physical changes. Changes in physical appearance include wrinkles and age spots. Although weight often increases in middle age, it frequently declines after age 60 because of muscle loss. Blood pressure often rises in older adults, although it can be treated by exercise or drugs.

Normal aging also involves some bone tissue loss from the skeleton, but in some instances the loss can be severe, as in osteoporosis. Almost two-thirds of women over 60 are affected to some degree by osteoporosis. Estrogen replacement therapy can reduce bone loss for women, and a program of weight lifting can help.

Chronic diseases--characterized by a slow onset and long duration-are rare in early adulthood, increase in middle adulthood, and become more common in late adulthood. The most common chronic disorder in late adulthood is arthritis; the second-most-common is hypertension (high blood pressure).

These physical deteriorations may sound rather dismal. However, a substantial portion of older individuals are still robust and active, even over the age of 85. Physical exercise slows the aging process and helps older adults function in society. Mental exercise, a positive outlook, and social support also appear to contribute to physical well-being in old age.

1		The number of years that an average person born in a particular year can expect to live is the .
2		The maximum number of years that any individual can live is the life
3		The theory of aging that states that people age unstable oxygen molecules damage DNA and other cellular structures is the theory.