AGE, OF COURSE, imposes a number of penalties upon us: We lose muscle tone. Our reflexes slow. We require reading glasses. Our hair thins. We lose an appreciation for the profundity of rock lyrics. The question -- a crucial question for those of us who value our wit as much as our abs -- is whether a decline in intelligence belongs on this dispiriting list. The best evidence that it does has always been the effect aging has upon those who depend most on high-intensity mental gymnastics: chess masters, for example, or lyric poets, or inventors, or mathematicians, or, to choose the classic example, physicists.

Paul Dirac, who won the Nobel Prize in physics at the age of thirty-one for work he completed when twenty-five, put that evidence into verse:

Age is, of course, a fever chill That every physicist must fear, He's better dead than living still When once he's past his thirtieth year.

Newton had invented calculus and worked out the basics of his laws of motion and his theory of gravity before he turned twenty-five (though the Principia Mathematica did not appear until he was forty-four). Einstein's general theory of relativity, probably his greatest achievement, was published when he was thirty-six; however, Einstein had published his special theory of relativity, along with the work on the photoelectric effect for which he would win the Nobel Prize, when he was only twenty-six. "A person who has not made his great contribution to science before the age of thirty," Einstein himself once stated, rather bluntly, "will never do so." Heisenberg came up with quantum mechanics at age twenty-three, the uncertainty principle when he was twenty-five.

Great work, blessedly, appears to get done later in life in fields where that work goes slower and seems less dependent upon sudden bursts of brilliance: history, for example, or geology, or writing novels. Tolstoy completed War and Peace and wrote Anna Karenina in his forties. Dostoevsky finished The Brothers Karamazov when he was fifty-nine. Goethe published The Sorrows of Young Werther when he was in his twenties, but the second volume of his long, dramatic poem, Faust, was completed as he entered his eighties.

Still, in fields where, as in physics, a mind must be able to juggle very many concepts very rapidly that disturbing drop off is hard not to notice. Bobby Fischer won the world chess championship at the age of twenty-nine and surrendered it at thirty-three. Anatoly Karpov took over that title at twenty-four and held it until, at thirty-four, he was defeated by twenty-two-year-old Gary Kasparov. (Kasparov himself was thrity-four when he lost to a ridiculously young IBM computer.)

New technologies also tend to leap out of young minds. Thomas Edison invented the phonograph at the age of thirty, the electric light at thirty-two. Guglielmo Marconi had produced a working "wireless telegraph" by the time he was twenty-one; Philo Farnsworth was the same age when he assembled a functioning electronic television. Alan Turing first raised the possibility that a machine might perform logical operations -- the possibility of designing, in other words, a computer -- in a paper published when he was twenty-five.

Robert Kanigel of MIT, author of a wonderful biography of a mathematician, The Man Who Knew Infinity, reports that in that field "it's part of the culture, it's accepted, that a mathematician in later years -- after the late thirties or forties -- is washed up." This sense that we hit a chronological, cognitive "wall" is certainly part of the culture in theoretical physics, too, though it can be hard to get physicists, a group not prone to sharing anxieties, to admit it.

"Nobody likes to think that they have gone past their peak," notes Alan Lightman, who was an astrophysicist at Harvard when he did admit to thinking precisely that in a melancholy 1984 New York Times "About Men" column. "It's a very unpleasant feeling," he explains. "Physicists, like athletes, are people who are superior to the average person at something. They get a lot of their self-identity from that special ability. It is very painful to come to the realization that you're not as good at it as you once were." (Lightman himself moved on to an enterprise at which it seems possible to be good longer. He published a well-regarded novel, Einstein's Dreams, in 1993; another novel, The Diagnosis, has just appeared.)

"None of my fragile childhood dreams, my parents' ambitious encouragement, my education at all the best schools," Lightman had written in that Times piece, "prepared me for this early seniority, this stiffening at 35."

Physics programs certainly don't offer seminars on how to cope with mental "stiffening at 35." "No one mentions it explicitly," acknowledges one somewhat older graduate student at Princeton, "but because the mythology of the famous physicists who did their work young is so strong, it is definitely in the atmosphere." It doesn't help that among the new graduate students in physics at Princeton it is not uncommon to find a teenager.

ANECDOTES ABOUT BRILLIANCE in twenty-somethings and impressions of stiffness in thirty-, forty- or fifty-somethings do not, however, prove the existence of that chronological wall. Statistics have, therefore, been deployed, studies made. The best is probably a "historiometric" survey of more than two thousand well-known scientists by Dean Keith Simonton. It appears to take us as close as we can currently get to an answer to the question of whether and when scientific ability declines.

Simonton, the most respected researcher in this field, concludes that individuals in physics, mathematics, and technology make -- on average -- their "best contributions" to their fields just before the age of forty. They make their "last major contribution" to those fields -- on average -- in their early fifties. Scientists in biology and the earth sciences, on the other hand, tend not to make their "last major contributions" until their late fifties.

The contrast with scholars in subjects like history and philosophy is more dramatic. Simonton's research shows that -- on average -- "they don't even reach their career peaks until they enter their sixties." (One assumes that individuals who write for online magazines will enjoy a similarly ascending trajectory.)

So there does indeed appear to be something about the ability to get the scoop on subatomic particles, dive into equations and give birth to new machines that fades -- relatively fast. That chronological wall does seem to exist, though we aren't likely to hit it quite as early as Lightman, not to mention Dirac, feared.

And before abandoning the blackboard, the aging physicist should remember that there have always been those whose careers are not average. Brian Greene of Columbia, who at thirty-eight says, "I'm hopeful that my best work is yet to come," points to the career of Edward Witten, eleven years his senior. Witten remains the leading figure in what is now the hottest field in physics, string theory. And there are more dramatic exceptions. Silvan S. Schweber of Brandeis is among those who cite "the remarkable Hans Bethe at Cornell, who is now working in a narrow area but still doing cutting-edge stuff in his nineties."

Exceptions. However, Schweber himself switched from physics to the history of ideas when he was about fifty. "I felt I was no longer being a creative physicist," he recalls. "Age was a part of it in the sense that there were striking new developments in my field, quantum field theory, but somehow I couldn't see them."