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I used to run with a guy who was unhappy with the way his performance had deteriorated over the years. In his early 20s, he said, he had been super-fast. A couple of decades later and about 20 pounds heavier, he had lost that amazing speed.

“Too many miles on the tires,” he would say. His idea was that if you start racing when you are young, you will be worse in middle age than if you started fresh when you were older.

But is it true, and if so, how does it happen? Do athletes accumulate injuries, for example, or just get mentally fatigued after competing nonstop for decades?

Personal Best Gina Kolata on exercise.

There are no definitive data on this question, but there are some suggestive findings, said Dr. Vonda Wright, an orthopedic surgeon and exercise researcher at the University of Pittsburgh.

Dr. Wright’s study of senior Olympians — athletes age 50 and older who participated in the National Senior Olympic Games, a track and field event — found what she considers a surprisingly small rate of decline in performance until age 75: just a few percent a year in their times. After that, though, the athletes slowed down considerably.

She asked the athletes when they began participating in sports. In her survey, 95 percent said they were active in sports when they were teenagers and 85 percent said they were active as young adults.

But the survey did not ask what sports they played when they were younger — the same sports or different ones from those they were competing in now — or when they began to compete (it is likely that many of the women, growing up before Title IX, did not compete when they were young). Both factors bear on whether late-blooming athletes have an advantage as they get older.

Hirofumi Tanaka, an exercise researcher at the University of Texas at Austin, has some data that bear on the question, albeit obliquely. He and his colleagues measured the maximum oxygen consumption, or VO2 max, of 153 men ages 20 to 75. Because VO2 max describes how much oxygen can get to muscles during exercise, it is measure of how well a person can perform. Sixty-four of the men in his study were sedentary, and 89 were trained endurance athletes.

The results were something of a surprise. The endurance athletes had a greater VO2 max than sedentary men of the same age, but this measure also declined more swiftly with age among the athletes. And although Dr. Wright may be right that each year performance times decline only a few percent, that steady decline year after year takes its toll.

In their 20s and 30s, the endurance athletes could run 10 kilometers, or 6.2 miles, in about 36 minutes. In their 40s they were almost as fast — 38 minutes. But in their 50s, the men averaged about 44 minutes. Those older than 60 took about 53 minutes to run that distance.

If sedentary men suddenly took up an endurance sport, could they match or even surpass the longtime athletes? Without years of cumulative injuries, the inevitable price of any long-term and rigorous exercise program, might the newer athletes have the edge?

“This is a good question that nobody has addressed in the past,” Dr. Tanaka said. But, he added, there is plenty of anecdotal evidence that older elite athletes often were not athletes when they were young.

Kozo Haraguchi, a former world record holder for his age group in the 100-meter sprint, ran it in 22.04 seconds when he was 95 years old. Mr. Haraguchi broke his own record two months later, with a time of 21.69 seconds. Yet this astonishing sprinter did not even start jogging until he was 65. He did not start sprinting until he was 76.

“Most of the masters distance runners who compete at a high level are also slow starters,” Dr. Tanaka said. In his study of endurance athletes and sedentary men, the average age at which the distance runners who were older than 60 had taken up the sport was around 40.

But might this simply reflect the fact that so many longtime athletes retire when they are still young, before they stop winning races, leaving the field to novices? If elite athletes like the swimmer Dara Torres, 45, and the marathoner Joan Benoit Samuelson, 55, choose to stay in the game, to continue to train hard and compete, then maybe it will turn out that long years of competition hold a big advantage for older athletes.

Or maybe not.

At this point, Dr. Tanaka said, “nobody has the answer.”