MONDAY, Feb. 11 (HealthDay News) -- Among Americans living to the ripe old age of 100 and counting, it is the ability to delay the onset of disability, and not the onset of disease, that seems to secure a long life.

A new study reveals that 32 percent of centenarians struggle with age-related illness for 15 years or more before hitting the 100 mark. Yet mental or physical disability is no more prevalent among this group than among centenarians who stave off disease until later in life.

"One would have guessed that to get to extreme old age, you'd have to avoid or delay disease," said study co-author Dr. Thomas Perls, director of the New England Centenarian Study at Boston Medical Center. "But some very old people with significant illness still live independently. And the message here is that one shouldn't jump to the conclusion that disease equals disability."

Perl's study is published in the Feb. 11 issue of the Archives of Internal Medicine.

In it, he and his colleagues noted that men and women over the age of 85 are the fastest-growing segment of the American population.

A 2001 U.S. Census Bureau report further reveals that, as of 2000, a little more than 50,000 centenarians were living in the United States -- roughly one in every 5,578 people. By 2050, this figure will rocket up to 834,000 Americans living past 100.

To explore this phenomenon, the authors analyzed health history questionnaires regarding 739 men and women between the ages of 97 and 119. A little more than 70 percent of the participants were women, and almost all were white.

Almost one-third of the centenarians indicated that they had initially developed at least one age-related disease before the age of 85.

Yet the researchers found that by the time they reached the 100-year mark, these so-called "survivors" of heart or lung disease, osteoarthritis, diabetes, high blood pressure, dementia, and/or Parkinson's were generally no more disabled or functionally dependent than seniors who remained disease-free until after age 85.

Stark gender differences were apparent, however. While noting that women are far more likely to reach 100 than men, the authors found that male centenarians function significantly better physically and mentally than female centenarians.

Among "survivors" alone, 72 percent of men were considered functionally "independent", compared with 34 percent of women. And among all centenarians, 67 percent of men had normal or mildly impaired cognitive function, compared with just 42 percent of women.

The researchers floated the explanation that men are generally far less resilient than women, so that those men who do make it to 100 are the cream of the crop.

Overall, Perls and his team concluded that for many elderly people, disability, rather than disease, determines longevity.

"This is just a first step of what I think is a pretty cool finding," said Perls. "And what we need to do now is try to discover what enables these people to markedly delay their disability. We have to go over all the possible factors -- socioeconomic, genetic -- and see what plays a role. Where do they get their functional reserve, resilience, capacity? We don't know the answers yet."

Meanwhile, Dr. James S. Goodwin, director of the Sealy Center on Aging at the University of Texas Medical Branch in Galveston, expressed little surprise at the findings.

"What this study shows is that disease is not the best way to assess the health of older people," he said. "If you live long enough, you're going to pick up a lot of them -- diabetes, high cholesterol, high blood pressure, heart disease -- but, by themselves, they don't immediately make people feel bad."

"But when people go on to become disabled, that's the bad sign," he added. "Because it's disability that interferes with your life and your ability to thrive -- to be physically and mentally able to reach your potential. So really, these things we call diseases could be thought of as risk factors for disability. Because when people become disabled, that's when they become truly sick. And that's when they stop living long."

A separate study in the same journal from Brigham and Women's Hospital in Boston revealed that men who do not smoke and practice a healthy lifestyle -- involving regular, vigorous exercise and weight control -- are more likely to live to age 90 and beyond.

The finding was based on questionnaires completed in the early 1980s by almost 2,400 men (average age 72), 40 percent of whom went on to live past 90. The analysis also suggested that men aged 90 plus have better physical and mental function then men who die at a younger age.

More information

For additional information on centenarians in the United States, visit the U.S. Census Bureau.