To some it's the Space Age, to others the Information Revolution, the Third Wave or the Age of Aquarius. But to Pulitzer Prize-winning psychiatrist Robert N. Butler, the 20th century is "The Century of Old Age."

"In 1900, the average life expectancy was 47, and now it's 73," says the 55-year-old director of the National Institute on Aging. "That's a spectacular 26-year gain in the past 80 years. This is the first century where people have a greater-than-50-percent chance of living out a natural life course. Eighty percent of today's newborns will live until their sixties. In 1830, only one-third did."

Add to this increased longevity "the graying of the baby boomers," and the result is, he says, "an aging explosion." By 2010, the number of "elder people" is expected to increase by almost 50 percent. By 2030, nearly one in five Americans will be 65 or older.

Which is why Butler has spent most of his professional life probing an area he admits most people dread--the process of aging and death. His goal: "For all of us--those who are old and those who will one day be old--to work to build a compact between the generations that will assure every person a truly decent and dignified old age."

Butler's fascination with "why people die" started at age 7 when the grandfather who raised him "disappeared suddenly. There was no talk, no funeral--only a protective silence that was more confusing than shared sorrow." He vowed to be a doctor in the hope that the right kind of medicine could help people live longer.

In 1976, Butler won widespread recognition as the nation's Gerontology Guru when he reported for work as the first director of the new National Institute on Aging on the same day his much-acclaimed book, Why Survive? Being Old in America, won the Pulitzer Prize for nonfiction. Old age is too often a tragedy, he told reporters, because "we have shaped a society which is extremely harsh to live in for the old."

Six years later, he is preparing to take the next step in his quest to improve the lot of America's elderly. On Sept. 1, he is leaving NIA for New York's Mount Sinai Medical Center, where he will head the nation's first geriatrics department established in a medical school. Looking back over the progress in the field of aging since the eventful afternoon--dubbed Butler's Big Day--he says, "there have been both good and bad changes."

First among the positive changes, he says, is a "somewhat greater awareness" of ageism, a word he coined for "all discrimination connected with being old." This has been helped by the Age Discrimination Act (which went into effect in 1979), the "gray-power movement" and "the very fact that there's been a National Institute on Aging . . . because there is now new knowledge related to old age.

"Specifically, we've learned a great deal about senility, which is certainly one of the most distressing conditions to confront people in old age. We've learned first that it's not a given, second that it's not arteriosclerosis, third that it's in fact predominently due to a disease called Alzheimer's disease. And there are now some very specific, major . . . leads for potential interventions in the next decade, maybe, in being able to prevent or treat what probably is the number-four or number-five cause of death.

"Another good thing that's happened is the beginnings of the development of geriatric medicine in the United States, so that older persons can feel that their doctors and nurses will be taught in medical schools by people who've made a life's work out of understanding the particularities of aging and the diseases and disabilities associated with it."

Currently, he says, about 50 of the country's 126 medical schools have "something going on in geriatrics." But just 2 percent of the eligible medical students take these elective courses.

One reason for the scarcity of geriatrics training, he says, is the "general resistance" to the necessity of such a field. But much of this skepticism has been dispelled by "new, compelling reasons for developing the discipline," including the "demographic imperative . . . large health costs" and "new research and knowledge that gives us something to teach."

At the new geriatrics department he will head, instruction in the geriatric aspects of each medical area--such as pharmacology and physiology--will be woven throughout the medical school curriculum. Senior students will also be required to spend four weeks in clinical work.

"I don't favor a specialty in geriatrics," Butler says. "I think every physician should have a general knowledge of aging. But there should be a cadre of sub-specialists . . . to back up the regular doctors as consultants."

The Mount Sinai geriatrics department will also include a think tank and a research arm. "We need to look," he says, "at the social, economical and political aspects of aging."

Despite the positive progress in the field of aging, Butler notes, "We continue to have an awful lot of gloomy souls who rail about the terrible burden of old people. They forget the underlying reason for health costs in old age is that we have postponed or deferred the problems that used to occur early in life.

"There's still an awful lot of tension and uncertainty and fear about Social Security. If we lived in a truly primitive society, the family and village would make sure people would not have to be frantic about old age. But since we live in a very humane and sophisticated society, everybody over 60 is scared to death as to whether they're going to be able to make it."

Much of this aging anxiety results, he says, "from the basic, universal, underlying dread about our own personal aging and death--which many cultures experience." But in America, he says, this terror is exaggerated "by our superficial preoccupation with youth.

"We've had systematic evidence of all sorts of ageism in jobs, in medical care. There's a commericial about how awful it is to reach 30--the terror starts that early. That plays right into ageism."

This problem will be particularly pronounced for the baby boomers, says Butler, who is working on a book tentatively titled Generation at Risk: When the Baby Boom Goes Gray. Boomers "should all start thinking now about the strong psychological connection between their present selves and future selves.

"I don't mean to get obsessed by old age . . . But don't just talk about gray power. Be a part of gray power. Read about and pay attention to issues concerning the life cycle--Social Security, senility, nursing homes. Does the National Institute on Aging get enough funding? Forewarned is forearmed."

How, then, has his personal study of aging affected his ability to deal with his own advancing years?

"I've certainly begun to experience what older people must experience a lot. Of my resident class, nine of us are dead. When I learned about a friend's death recently, there was a sense that having thought and read about the life cycle had a kind of preparatory aspect to it. But it didn't make it hurt any less.

"I think that having knowledge is vital. It has its protective aspects. But I certainly hope, and don't think, it's so protective you don't experience things. Because then it becomes a kind of a wall instead of an appropriate cushion.

"I still think I'll be in for a lot of surprises. I think there are a tremendous number of secrets that you can only know when you're old, or you can only really perceive if you're old. And until you're there, you can't really make that leap over the chasm."