If medical science makes steady progress against the diseases that kill older people, then people 85 or older will make up 18 percent of the population in 100 years.

Such a dramatic change from the 1 percent figure of today is at least "conceivable," a study in the American Journal of Public Health concludes, and even without any medical progress, the percentage of people in that age group will more than double by the year 2080.

If, on the other hand, death rates drop by just 1 to 2 percent a year, then 40 percent of the 2080 U.S. population will be over 65.

"Delayed retirement will almost certainly be required to save Social Security from bankruptcy," predicts James W. Vaupel of the University of Minnesota. "If more of the elderly hang onto their jobs, however, promotional opportunities will diminish for the young . . ."

Vaupel and fellow researcher Ann E. Gowan discuss the prospect of an older America in their article, "Passage to Methuselah." Among their predictions:

Society will face the "major challenge" of letting old people continue to work while giving young people a chance.

People will work 28 hours a week from age 22 to age 82, taking off two months a year and an entire year every decade "for ongoing education."

Even people working until age 82 can still expect 18 years of retirement.

Liberal arts education will be especially important because it "helps people maintain an active interest in life."

Will such a change come about? Other predictions hold that life expectancy may never go above 82.5, much less to 100.

But if it does, the impact will be slow. "If death were eliminated tomorrow," the study says, "it would still take a century before there would be many 200-year-olds. So society will have time to adjust to the new demography."