The study of aging is still in its youth.

The National Institute on Aging, which funds much of the nation's biomedical research on aging, turned 10 last week, the youngest of the 11 research institutes that make up the Bethesda-based National Institutes of Health.

In the decade since NIA was established, the entire medical literature on aging has doubled, says James Birren, dean of the Andrus Gerontology Center at the University of Southern California.

That fast-growing literature has helped define and redefine the aging process, overturning stereotypes and debunking myths about the elderly. While growing old is still inevitable, it is no longer equated with inevitable illness, disability and dependence.

Dr. T. Franklin Williams, one of the nation's pioneering gerontologists and now director of NIA, envisions a day when Americans would finally be able to "think of older people as being people.

"We've got to break out of this mode of thinking that just because you're old, you're different from other people," says Williams. "I don't really think we are."

Experts on aging gathered last week in Bethesda to mark NIA's 10th birthday, celebrate the progress of those 10 years and call for renewed efforts "to see the life course whole."

The past decade has seen "a dramatic improvement" in both the amount and the quality of research on aging, says Dr. John W. Rowe, director of the division on aging at Harvard Medical School. "But I would hasten to say that the amount is still small, relative to the need, and we're still skating on thin ice."

When he and his colleagues visit geriatric patients in the teaching hospitals, "We still don't know the answers to 90 percent of the questions that the medical students ask us."

Perhaps the major struggle of the past decade, says Dr. Robert N. Butler, former NIA director, has been the "separation of age from disease."

New evidence shows that the aging process works very differently in different individuals, suggesting the possibility of altering the process or minimizing its adverse effects with proper care, preventive efforts and better social services for the elderly.

"You can no longer say: 'Oh yes, aging -- you can't do anything about that,' " says Butler, now chairman of the department of geriatrics and adult development at Mount Sinai Medical Center in New York.

But he says continued progress on aging is jeopardized by "systematic cutbacks" and "an extraordinary allocation of resources" by the Reagan administration and Congress.

NIH's entire budget, covering research funded by 11 member institutes, Butler pointed out, is about $5 billion -- the equivalent cost of three Trident nuclear submarines.

"Isn't a healthy American population a very important part of national security?" asks Butler.

The budget of the National Insitute on Aging alone, now about $140 million, should be more than tripled to the "half-billion-dollar range" by 1990, Butler says. That would allow for expanded research on such illnesses as Alzheimer's disease, an incurable form of memory loss that he said causes nearly half of all admissions to nursing homes.

Butler also charges the Reagan administration with having "refined the art of the politics of austerity" so that the burden of cost containment falls disproportionately on the poor, the sick and the aged.

"We're seeing a gentrification of the health care system," he says.

Two Sundays ago, Mount Sinai Medical Center admitted a confused 84-year-old woman who was nearly blind and had been accidentally setting fires in her apartment. She didn't need medical care, but there were no community-based services available for her. The hospital had no choice but to admit her, knowing reimbursement for her care will "probably be denied" because hospitalization is clearly inappropriate in her case.

"This is happening, and it's happening all over," Butler says.

Such issues have important social and political implications because of what has been called "the demographic imperative" -- lengthening life expectancy and the aging of the American population.

More years have been added to the average life span in this century than in all the centuries between pre-Roman times and 1900, says USC's Birren. Life expectancy of the average American born in 1900 was 45 years. Life expectancy at birth today is 71 for males and 78 for females.

Americans 85 and older are now the fastest-growing segment of the population.

Increasingly, researchers have tried to distinguish the physiology of normal aging from the effects of disease in the elderly, by studying large numbers of reasonably healthy people as they grow older.

The oldest and best-known of these studies is the Baltimore Longitudinal Study, now in its 28th year, which is following the aging process in some 650 men and nearly 400 women. The participants -- all reasonably healthy volunteers -- range in age from their twenties to their nineties.

Until very recently, "The conventional wisdom was that aging was sort of a downhill slide," says Richard Greulich, scientific director for NIA. But the Baltimore study has shown that healthy people have a remarkable capacity to adapt to aging and stay active.

It also uncovered a surprising physiological variability among old people -- more than in younger people.

"As people age," Greulich says, "they become more and more themselves." That contradicts a long-held view that the people who live to old age tend to be more like each other than a population of young people.

"We're on our way to an increased appreciation of variability in aging ," says George Maddox, a medical sociologist from Duke University. That variability offers the possibility of changing or preventing some of old age's vulnerabilities.

"The good news in gerontology," he says, "is that some of the bad news has been substantially wrong."

Evidence is mounting that age alone does not determine medical risk. Recent research has shown, for example, that otherwise-healthy old people are almost as likely as young people to survive open-heart surgery.

Ten years ago, says Harvard's Rowe, most doctors wouldn't even have thought of discussing prevention with an elderly patient. Today, there's a growing recognition of what Rowe calls "the plasticity of aging."

"It's remarkably modifiable," he says. Studies show that stopping smoking significantly improves health, regardless of age, and that getting 80-year-old women who live in nursing homes to exercise helps prevent mineral loss in their bones and reduces the risk of hip fractures.

But gerontologists cannot consider aging apart from its social context, warns Duke's Maddox. Despite all the research on medical risk factors, his advice is: "Do not go into late life -- if you can help it -- poor, ignorant or without a vital support system."

Gerontology, he says, is inextricably bound up with public policy issues.

"The problem we're working on is not just a scientific one but a political one," he says. "Science and politics are going to come together in aging."