Why do some people live to a ripe old age and stay healthy, while others don't?

The answer may lie in their genes.

New research examining the genes of centenarians suggests they have fewer genes associated with serious diseases such as Alzheimer's and heart disease. The result: they don't get sick as much as others who never reach that 100-year milestone.

But hang a second before you abandon all your healthy lifestyles. Granted, there's nothing you can do about your genes, but longevity also is affected by other factors over which you do have control.

The study was published this week in PLOS Genetics by researchers at Stanford University and the University of Bologna. It's the first one to identify several genes associated with long life, and the first to pinpoint their exact location on a chromosome.

Scientists behind another longevity study — the New England Centenarian Study — have discovered some common traits of people who are at least 100. They say most centenarians don't smoke, are not obese, and are able to handle stress better than others.

But, in a nod to the "good genes" theory of longevity, at least half the centenarians studied had close relatives who also hit the century mark.

Wondering about your own odds of becoming a centenarian? Go to www.livingto100.com and use the "Living to 100 Life Expectancy Calculator" made by the New England Centenarian Study folks.

"To Your Health" offers quick doses of health news several times a week.