While the Buck Rogers set has long pictured the 21st Century as an era of flashy, wrinkle-free technology in the hands of bounding youths in leotards, writer-scientist Isaac Asimov sees something else.

More likely, he says, it will be populated predominantly by old people using home computers to recycle themselves through a series of careers as technology makes their old jobs obsolete.

The result-if we're lucky-will be an age of creativity, Asimov said today. On the other hand, maybe it won't.

Asimov, billed as "the American Jules Verne," laid out his vision of utopia here for about 400 leaders of business, labor, government and the social sciences gathered to ponder "Working in the 21st Century."

The primary ingredients in the Asimov recipe call for reduction of the birth rate worldwide and increased automation. Just as the industrial revolution freed 9o percent of the population from "grubbing in soil" for a living, he said, computers will, by doing the world's routine and boring chores, "spare the brain from work that is beneath it."

"People will no longer be judged by the number of units they turn out, but by how much they increase the joy of the world," Asimov declared.

Though clad in pinstripes and not a leotard, the author leaped deftly over such questions as paring the world birth rate, solving the energy shortage and dealing with clerks, mailmen and bank tellers computerized right out of the work force.

Other speakers tackled those questions but the only consensus about the 21st century to emerge from the day, as public commentator James C. Lehrer put it, was that "George Meany will not be with us . . . "

The future, as outlined by the certified experts, "frankly scares the living daylights out of me," Lehrer told the audience. "Not for me but for my grandchildren, although I'm not sure Dr. Asimov would allow my children to have any."

Other at the podium ranged from historian-economist W.W. Rostow to William W. Winpisinger, the fiery president of the International Association of Machinists and Aerospace Workers, who in his speech compared the nation's Business Roundtable to the Soviet Politburo.

Wednesday the two-day conference will hear speakers ranging from pollster Louis Harris to Stewart Brand, editor and publisher of the Whole Earth Catalog.

In Asimov's view, as social and technological changes lower the birth rate and increase life expectancy, we can expect to have a more and more aged society.

This pattern has already been developeing in the United States for a century, he noed. The aged will become an "increasingly vigorous voting block" and because we all expect to join this bloc eventually we will be unlikely to oppose it.

The result of all this could be a society which revolves almost exclusively around pension plans, social security and medicare, where "the backs of the young are weighed down and eventually broken by aged people considered drains on society."

Instead of dying in a [population] explosion, Asimov said we might die "with a whimper of old age."

However if we mend our present ways and make the right choices, he said, life can be beautiful even in a gray century. Our current view of the elderly is an ignorant one, he said, because historically, there haven't been many of them around."

Society has adopted a "bigoted" approach to education, for example, limiting it to a rite of passage for the young only, he said. Getting out of school is "a sign of growing up," and once out a young person has a tendency "to want never to read another book or have another thought."

Thus, he said, we ourselves create the old people we feel are a drain on society.

Reactions to the discussions varied from grumbling about government regulations that "slow down progress" to the obsevation by Fred Panzer, a Tobacco Institute executive, who speculated the new order would see more conferences like this one. The endangered species of the computer age, he said, will be PhDs like those on the panel here, "And nobody knows how to fight for their jobs like they do." CAPTION: Picture, Writer-Scientist Isaac Asimov talks of 21st century minds freed by computers of doing work beneath them.