Mary Wittenberg, the 44-year-old president of New York Road Runners, is a fast, strong and experienced runner. But she races best, she says, when she runs just behind Witold Bialokur. He can run 10 kilometers, or 6.2 miles, in less than 44 minutes and he is so smooth and controlled.

“He’s like a metronome with his pacing,” Ms. Wittenberg says. “I am often struggling to keep up with him and it’s a good day when I do.”

While Mr. Bialokur’s performance would be the envy of most young men, he is not young. Mr. Bialokur is 71.

It is one of the persistent mysteries of aging, researchers say. Why would one person, like Mr. Bialokur, remain so hale and hearty while another, who had seemed just as healthy, start to weaken and slow down, sometimes as early as his 70’s?

That, says Tamara Harris, who is chief of the geriatric epidemiology section at the National Institute on Aging, is a central issue that is only now being systematically addressed. The question is why some age well and others do not, often heading along a path that ends up in a medical condition known as frailty.

Frailty, Dr. Harris explains, involves exhaustion, weakness, weight loss and a loss of muscle mass and strength. It is, she says, a grim prognosis whose causes were little understood.

“It means that some people spend a long time in a period of their life where they have lost function,” Dr. Harris says. “People find that very distressing, and there is a tremendous health care cost.”

Now, though, scientists are surprised to find that, in many cases, a single factor — undetected cardiovascular disease — is often a major reason people become frail. They may not have classic symptoms like a heart attack or chest pains or a stroke. But cardiovascular disease may have partly blocked blood vessels in the brain, the legs, the kidneys or the heart. Those obstructions, in turn, can result in exhaustion or mental confusion or weakness or a slow walking pace.

Investigators say that there is a ray of hope in the finding — if cardiovascular disease is central to many of the symptoms of old age, it should be possible to slow or delay or even prevent many of these changes by treating the medical condition.

A second finding is just as surprising to skeptical scientists because it seemed to many like a wrongheaded cliché — you’re only as old as you think you are. Rigorous studies are now showing that seeing, or hearing, gloomy nostrums about what it is like to be old can make people walk more slowly, hear and remember less well, and even affect their cardiovascular systems. Positive images of aging have the opposite effects. The constant message that old people are expected to be slow and weak and forgetful is not a reason for the full-blown frailty syndrome. But it may help push people along that path.

Still, it is a view that can lead to blaming the victim, and some scientists at first resisted it. Now, though, more and more say they have been won over by an accumulating body of evidence.

“I am changing my initially skeptical view,” says Richard Suzman, who is director of the office of behavioral and social research programs at the National Institute on Aging. “There is growing evidence that these subjective experiences might be more important than we thought.”

The Walking Test

Eleanor Simonsick’s initiation into the unrecognized debilitations of aging came with a research study she helped set up. The question was whether older people who are relatively vigorous are also longer-lived. As an epidemiologist at the National Institute on Aging, she thought it was time to ask that in a rigorous way.

So she and her colleagues recruited 3,075 apparently healthy people in their 70’s who said they could walk a quarter of a mile with no trouble and climb a flight of stairs. Each was asked to walk up and down a corridor 10 times, for a distance of a quarter mile, maintaining their pace and not stopping to rest.

A quarter of them could not do it. And it was not just a matter of age. The average age of those who could do it was 73. So was the average age of those who could not. Dr. Harris explained: “I believe most people can amble. But we were asking them to walk as quickly as they could without stopping. That’s what people couldn’t do.”