We were one hour into the Picasso exhibit when we stopped in front of a cubist painting called "The Accordionist."

Behind us were five huge rooms full of gorgeous Impressionist children and massive seated women, of perfectly reproduced realism, of shattered forms of revolutionary cubism.

At the bottom of this one picture, we checked the date: 1911. He had done all this before his 30th birthday.

The age impressed me more than it did the 12-year-old next to me. We live on opposite sides of that dividing line. Yet we were both struck by the volume and versatility of Picasso's life work in this exhibit.

Surrounded, even overwhelmed, as we moved among the 900 Picassos that have taken over the Museum of Modern Art for the summer in a massive retrospective, it was obvious why this man still dominates art the way Shakespeare dominates literature or Mozart dominates music.

It is said that when Picasso was a teen-ager, his artist-father gave the boy his own palette, brushes and colors and never painted again. It is known that when he died at the age of 91, Picasso was arranging for a show of his latest work. In between, he was astonishingly productive.

Here was a man who produced some 13,000 to 14,000 canvases, 100,000 prints or engravings and 34,000 book illustrations. He worked in virtually every medium from stage sets to ceramics, ranging back and forth from one to the other with as much energy as genius.

Yet as we wandered through the last 30 years of his life, you could see it all slip. The exhibit kindly excludes the commercial peace doves and greeting-card poster art of the last years.

But still, it is easy to see the versatility turning frenetic, the search turning downhill. There is even a sense that perhaps he began to imitate himself -- not just to create, but to create "Picassos."

No, there is nothing bad on these walls. The worst of this artist is very, very good. But winding down through his age and out again onto 54th Street, it was hard not to wonder what it was like to be Picasso at 70, or 80, or 90, competing with Picasso at 40.

What is it like to keep working in the present while your past has already been written into history books? What is it like to compete with your own best?

It is something that I've thought of before. I've thought of it whenever Tennessee Williams turns up in the news, alive but rarely well, writing poorly in comparison to his own brilliant retrospectives. I've thought of it when Frank Sinatra goes on stage, all blue eyes and strained vocal cords. They are pale versions of themselves.

Living in your own shadow is a problem of aging athletes and beautiful women and artists and actors and, to an extent, all of us.

The American ideal is that people should quit with the gold medals around their necks and the stars on their doors. We want them to stay on top or move on. We want to laurelize them like Jesse Owens or ignore them like Mark Spitz. We hope that, like Beverly Sills, they will "move on" at the right moment, off of one stage and onto the next . . . before their voices crack in public.

There are very few ways for our stars to retreat gracefully back into the chorus line. We live in such an achievement-oriented world that anyone who is not doing his or her best, breaking records, going onward and upward, is somehow or other failing.

We feel saddened that Joe DiMaggio sells coffee-makers and uncomfortable that Willie Mays "stayed too long." Few of us know quite how to deal with the man or woman who "used to be" somebody.

Picasso was hardly a failure in his later life. He refused to be canonized. He refused to rest on his laurels. He chose productivity. He got up in the morning, nearly paralyzed by pessimism about his own ability, and went to work.

There is something, not sad but remarkable, in this refusal to "act his age" or to retire gracefully. Surrounded by his own collection of his favorite cubist work, he must have known his limits. But out of compulsion or conviction, he kept working.

"Creation," Picasso said, "is the only thing that interests me." So for 91 years, he did something remarkable. He stayed interested.