By the year 2020, one in six Americans will be older than 65. This will affect social, economic and health-care areas.

But what exactly is aging? In his book "The American Geriatric Society's Complete Guide to Aging & Health," Mark E. Williams, M.D. defines aging as "a progressive, predictable process that involves the evolution and maturation of living organisms." Although the physical aging of our bodies is unavoidable, people age at widely different rates. Genetics — what you inherit from your parents and other ancestors — has a major influence on how you age, along with environmental factors and lifestyle. For example, if your parents both lived into their 90s, and if you get plenty of exercise, practice good nutrition and handle stress effectively, you'll probably live longer than the average sedentary, stress-ridden, junk food-addicted couch potato whose mom and dad died young. But there's no guarantee, of course.

These facts about aging and life expectancy have been known for some time. But new discoveries about the elderly are changing our long-held myths. We now know, for example, that disease and old age don't necessarily come together. New research suggests that Alzheimer's disease, for example, is not a natural element of the aging process. Moreover, evidence indicates that older adults are much healthier now than people of the same age were before them. The aging body seems capable of much more than has been widely acknowledged.

There's even more good news about aging. Except for extremely old people, life satisfaction does not seem to decrease with aging, despite some of the problems associated with old age, such as declining health, diminishing finances, loss of spouse and friends, reduced mobility and activity, and loss of self-esteem.

These revelations about aging and life expectancy come from two sources: geriatric research and geriatric medicine. Both fields are complex because they include so many variable factors. Geriatric researchers, or gerontologists, as they are also called, study factors that influence aging including genetic factors, environment, lifestyle, and changing demographic patterns. Researchers also get valuable insights into human aging by studying aging in animals, although extrapolations of these findings from animals to humans are frequently not valid.

Geriatric medicine, the other principal source of information about aging, is a medical specialty that focuses on the health care of older persons. Geriatricians must be certified in internal medicine or family practice and have additional qualification in geriatric medicine.

As America's population continues to age, expect more extraordinary discoveries about the aging process and the end of ancient stereotypes about old folks. As Ken Minaker, M.D., chief of the Geriatric Medicine Unit at Massachusetts General Hospital and associate professor at Harvard Medical School, observes, "Aging is a new field driven by demographics as much as by scientific discovery. Today's elderly are pioneers of a new kind of aging. The next century will be dominated by the concerns of the elderly."