Firstborn children of women younger than 25 are twice as likely to defy the average life span and go on to live beyond 100, according to a new study.

Leonid A. Gavrilov and his colleagues at the University of Chicago's Center for Aging have relied on a wealth of Internet data - from genealogy Web sites to federal death indexes - to study centenarians to figure out why so many firstborns seem to outpace their younger siblings in the longevity race.

Although there is no clear answer yet, scientists believe the phenomenon may be related to the physical youthfulness of young mothers and the eggs they produce.

The study results could have implications for the current generation of women who are pushing back motherhood, researchers say.

The scientists add that early-life factors may be important clues to age-related illness and longevity.

For the latest research, which was to be presented yesterday at the Gerontological Society of America's annual meeting in Texas, the scientists gathered information from people who have lived beyond 100 and studied a number of childhood variables to see whether they could explain this effect. They compared genealogy records to death indexes to make sure the information was accurate.

They studied family histories of 198 centenarians born in the United States between 1890 and 1893 and reconstructed their family records using U.S. Census data and information on the age at death from the Social Security Death Index.

They looked at survival statistics of these people compared to others born during the same time who had older siblings.

They also compared a group of firstborns to their siblings.

They found that firstborn children were 1.7 times more likely to live to this ripe old age.

The question is why.

They studied a variety of factors and what stood out strongly was that the age of the mother was a strong predictor of longevity.

The age of the father did not seem to make a difference on the life span of his progeny.

"The protective effect of being a firstborn is driven mostly by the young maternal age at the person's birth," Gavrilov said.

So what causes this exceptionally long survival?

Besides the youthfulness of the eggs, another theory, still unexplored, is that younger women haven't been exposed to as many viruses and diseases as older women, which in turn makes for a healthier uterine environment.

"It's good news for me," said Dr. Robert Butler, president and chief executive of the International Longevity Center in Manhattan and former director of the National Institute on Aging.

Butler was an only child whose mother was 23 when he was born.

"It's very interesting. We need to see more studies to replicate it," Butler said.

The study was funded by the National Institute on Aging and the Society of Actuaries.