Rep. Claude Pepper (D-Fla.), who is as old as the 20th century and has no intention of retiring until the 21st, celebrated his 84th birthday with a gala dinner last October at the Sheraton Washington Hotel. Among the guests were 40 fellow octogenarians, including Bob Hope, Anthony Quinn and Helen Cronkite (Walter's mother).

And a kid named Edward L. Bernays.

Bernays is, among other distinctions, the father of public relations, the nephew of Sigmund Freud and, though it hardly seems to matter, 93 years old.

As Pepper's guests entered the hotel, Marine escorts led them downstairs to the ballroom, lending a steady arm of support. Bernays started down the staircase, a well-intentioned Marine clutching at his arm.

Suddenly, without warning, Bernays broke away from the Marine and dashed down the stairs on his own. The other guests, dumbfounded at first, broke into uproarious applause.

"I decided I'd show people old age isn't necessarily debilitating," Bernays recalls.

"That guy," says a Pepper aide, "flew down the stairs. I mean, forget it. The poor escort was running after him, but never had a chance."

Age 65 is less and less a watershed for American workers. It holds less magic, less terror, less legal sanction.

For about two decades, more and more workers have been retiring before 65. More recently, since mandatory retirement at 65 was overturned by Congress in 1978, a smaller but also-growing group has been staying on the job past 65.

Meanwhile, Americans are living longer. The fastest-growing age group in the population comprises people over 85. In 1900 there were 123,000 Americans 85 or older; today, about 2.5 million. By the year 2050, the Census Bureau estimates, there will be more than 16 million over 85, accounting for more than 5 percent of the population.

Typical retirement is no longer just a few work-free years at the end of a career. A 65-year-old male today can expect to live to at least 80. For a growing number of Americans who retire early, retirement lasts as long as career.

"Retirement is no longer a luxury. It is now an institution -- as much an expected part of a life course as family, school or work," says "Aging America," a report prepared by the staff of the Senate Special Committee on Aging.

In 1900, the average male American had a life span of 46 years, of which 1.2 years, or 3 percent, was spent in retirement. By 1980, the average male had a life span of 69 years, of which 14 years, or 20 percent, was spent in retirement.

The American Association of Retired Persons (AARP) now has about 18 million members. Last year it added an average of more than 8,000 new members every day.

Demographic changes are undermining stereotypes and blurring a lot of traditional distinctions: between work and leisure, career and retirement, middle age and old age, health and illness.

"You can no longer meaningfully generalize about older people," says John Rother, an associate director of AARP, "any more than you can about people under 30."

In a national survey of people 55 and over by AARP in 1982, 74 percent of those still working said they'd prefer to continue working in some capacity after retirement, even if they could afford not to.

"There are a lot of older people -- surprise -- who want to work," Rother says. "They just don't want to work in the same job they've had up to retirement."

Retirement never occurred to Edward Bernays, who came to this country from Vienna as an infant in 1892.

He worked for the Committee on Public Information during World War I and attended the Versailles Peace Conference with President Woodrow Wilson in 1919, before returning to New York to found a public relations firm that revolutionized the field.

He still has clients. He still gets up at 5 a.m. every day, still works till 5 p.m.

When New York Mayor Robert Wagner asked Bernays to chair a senior citizens committee 30 years ago, "I didn't know what a senior citizen was. So I went to Columbia and studied up on geriatrics."

Today, he lives in Cambridge, Mass., where he is cochairman of the Committee for Economic Justice for Older Workers; honorary chairman of Careers for Later Years, a group that helps older workers find jobs; and chairman of a citizens advisory committee to the Massachusetts state Department of Elder Affairs.

Americans, Bernays says, are obsessed with chronological age. They assume, for instance, that because he is 93 years old, he needs help walking down stairs. But last year, Bernays' doctor told him his physiological age was 61.

And he's getting younger: "The year before, I was 62."

The growing irrelevance of chronological age is one of the most important social trends since Social Security was enacted 50 years ago, says Wilbur Cohen, former secretary of Health, Education and Welfare.

"If you ask what's the biggest thing that's happened in the 50 years I've been working on this," says Cohen, now a professor at the Johnson School of Public Affairs at the University of Texas, "it is that people want to do something if they're physically able, no matter how old they are."

A series of factors -- increasing life span, improved health, growth of private and public pensions, proliferation of service jobs and changing social attitudes -- has challenged the sanctity of age 65 as the onset of retirement.

When Social Security was enacted in 1935, eligibility was set at age 65 by a classic political compromise. The framers of the Social Security Act decided that 70 was too high, while 60 was too low and too costly. So they split the difference and picked 65.

(Despite a widespread myth, Cohen says, age 65 was not picked by Chancellor Bismarck when he designed the German social insurance program a century ago; in fact, Bismarck chose 70 as the age of eligibility, and it wasn't lowered to 65 until 1916.)

From Bismarck's time through the New Deal, Cohen says, the prevailing theory was that setting a universal retirement age for all workers was the fairest, least discriminatory approach.

Today, it's the opposite. Mandatory retirement age has been raised to 70, and many workers, including federal employes, are exempt. Retirement is now considered an individual decision.

Ultimately, Cohen says, "we'll come to the point where there's no chronological age when you have to retire. We've got 70 in the law now, but President Reagan is 74. Pepper is 84. Look at the Supreme Court."

At 71, Cohen himself is a college professor. ("I get paid half-time; I work full-time.")

When friends ask whether he is working or retired, Cohen, like a growing number of older Americans, gives the only sensible answer:

"Both."

The World War II generation -- parents of the baby boomers -- is starting to change the face of retirement in this country.

"What's happening," says Charles Longino, director of the Center for Social Research in Aging at the University of Miami, "is we're getting a group of people moving into retirement in the 1980s and 1990s who are going to have opportunities unheard of for the generation before them."

Compared with their parents, they are better educated, more mobile, healthier, better off financially and less willing to give up their accustomed style of life upon retirement.

"They've read more, they've seen more, they've been more places, they've argued about more," he says. "The world is smaller for them.

"It's going to look as though old people are getting younger."

He calls it "youth creep."

For example, Jane Fonda, at 47, is 17 years older than the woman standing next to the pitchfork-holding farmer in Grant Wood's classic painting "American Gothic."

President Reagan is another example of youth creep.

"Twenty years ago," Longino says, "if someone had said, 'In 20 years, you'll have a 74-year-old president who gets out and campaigns vigorously and rides horses and all that,' you would have said, 'Oh, come on now.' "

The main problem with the word "retirement" is that it implies not doing anything, as if, Longino says, there were only two conditions: work, and doing nothing.

"But that's not the real world. People want to stay active all their lives."

"People don't fall apart at 65 because they have a birthday," says Willis Goldbeck, president of the Washington Business Group on Health, whose members include more than 200 large companies around the nation.

He questions the value of thinking of life as consisting of three rigidly chronological cycles: education, work, retirement.

"Retirement is fine if you want a rest, to be 'done' for a time," Goldbeck says. "It's okay if you want to retire at 35 for two years to recharge, but retirement should be followed by rejuvenation, not by death."

Part of the problem with retirement is that it has long been sold to workers as a panacea, "the beginning of the good life," a just reward for all the awful years of labor.

"What a horrible way to start out in life," he says.

But as today's generation of corporate leaders retires, they are being replaced by substantially younger leaders -- a shift that he says will "change the nature of corporate America."

"You're beginning to see the 65- and 68-year-olds being replaced by the 45-year-olds," he says. "Are they going to be chairmen for 25 years? Highly unlikely.

"People will start to leave the role of chairman at 55 voluntarily."

Goldbeck says he's waiting for the day when a retired corporate chairman will run for Congress at, say, age 58.

"Right now, we have retired congressmen becoming corporate chairmen," he says. "That's going to turn around and go both ways."

Even blue-collar workers -- who tend to adopt the traditional retirement pattern of rest and reward for a generation of hard labor -- won't find 25 years of inactivity all that attractive, Goldbeck says.

"You can't play golf for 25 years, unless you're on the pro tour," he says, "and there aren't any retirees on the pro tour."

The most frequent reason people give for retiring is health, says Ron Abeles, deputy associate director for the National Institute on Aging. But in some cases, he says, that may be partly a rationalization, reflecting the fact that health is a socially acceptable reason for retirement.

Health care and its cost are a major worry of most older Americans, about one in five of whom has an income below the poverty level. Even excluding nursing home care, the average person over 65 accounted for three times the health costs as the average person under 65. Medicare, the federal health plan for the disabled and those 65 and older, covers less than half of the health care costs of the elderly and does not pay for nursing home care.

But a growing body of research suggests that, as a group, even the very old -- people over 85 -- are healthier, more independent and more involved than popularly believed.

A 1982 Gallup poll found that 56 percent of Americans 65 and older could name their congressional representative, the highest of any age group and more than double the figure for voters aged 18 to 24. The increasing political clout of older voters -- manifested in the growth of AARP and dozens of other Washington-based organizations -- refutes the notion that older Americans are politically indifferent or out of touch with the world.

A myth took hold that retirement was bad for health because some people suffer an immediate decline in health upon retiring.

But "no study has ever confirmed the hypothesis that retirement in itself leads to a deterioration of health," says David Ekerdt, a research sociologist with the Normative Aging Study at the Veterans Administration Hospital in Boston.

Ekerdt's team surveyed more than 700 healthy workers at retirement age and again three years later. It found a slight, gradual decline in health, consistent with normal aging, but there was no difference between workers who retired and those who continued to work.

Still, the myth persists that retirement is unhealthy. One reason, Ekerdt says, is that the experts most often cited -- doctors and academics -- tend to resist retirement themselves.

Also, work is still viewed as "the wellspring of identity and self-fulfillment." And retirement is such a landmark event for most people that they tend to attribute every subsequent change in their lives to it.

Anecdotes also promote the myth. Everyone has a story about somebody -- Uncle Fred or Grandma or an old coach -- who retired and "hasn't been the same since." When Alabama football coach Paul "Bear" Bryant died of a heart attack in 1983, less than a month after retiring following a victory in his last game, the Liberty Bowl, many columnists cited it as yet another example. They rarely pointed out that Bryant had had serious heart disease for years.

Whether one likes retirement or not depends partly on what one thinks it means. The word itself is unpopular, but nobody seems to have come up with a good substitute.

Even the government has never really defined "retiree," although it is usually understood to mean a person who was employed, but is no longer employed, and who is eligible for some kind of pension.

"I mean, John Havlicek retires and Johnny Unitas retires," says sociologist Ekerdt, "but everybody knows that's just a word, because they're going into successive careers."

Still active at 93, Edward Bernays hates the very word "retirement," which he says used to suggest a certain dignity but today means only that "the guy is old and decrepit and can't work any longer."

"Words in America," he says in disgust, "have the stability of soap bubbles."

But if that is so, there's hope for people who seek to change the definitions of retirement and old age.

Like Bernays himself. He hasn't lost his gift for public relations. Recently, he helped organize a protest in front of the State House in Boston and scored a PR coup by persuading a crowd of schoolchildren to join the Gray Panthers carrying placards saying "Don't Discriminate on the Basis of Age."

"It was on every TV station that night," he says proudly. "It was a lot of fun.

"So I keep on."