There are some 70,000 of them in the U.S. alone - people 100-plus years old, who make up the fastest-growing age group of Americans and whose numbers have tripled in just the last two decades.

Clearly, these "centenarians" can safely be called "over the hill," but how did they survive the mountain of age-related threats to reach the 100-year milestone?

Thomas Perls, M.D., founder of the New England Centenarian Study, has been working to unlock the secrets of the very old. "We have ingrained in us the idea that the older you get, the sicker you get, and so people would think these 100-year-olds must have every age-related disease under the sun and must certainly be on death's doorstep," says Perls. In fact, he explains, quite the opposite is true: These oldest Americans are still alive because they avoided these lethal age-associated diseases, such as cancer, heart disease and Alzheimer's.

Do Centenarians Have a Genetic Advantage?

Centenarians probably lack genes that predispose them to numerous deadly diseases, explains Perls, who is working with his colleagues to zero in on the human genes that dictate longevity.

His genetic research could "dramatically change our lives within the next seven to 10 years," he says. From his research could come the development of drugs that counteract disease-causing chemical pathways, making today's preventive surgeries seem primitive.

While Perls continues his search for the genetic "booster rockets" of the super-survivors, he encourages people to take their longevity into their own hands: Exercise, eat right and don't smoke, he says, and you greatly increase your chance of living in good health at least into your 80s.

And, adds Daniel Perry, executive director of the Alliance for Aging Research, try to be as positive as possible. As a rule, he points out, those who live to very old age "are engaged with life every day and don't have doubts about the value of life."

Know someone who's hit 100? Perls is searching for more study participants. To contribute to his research, call 888-333-6327 (toll-free) or go to The New England Centenarian Study.