Living for a century is no easy task. Researchers attribute long life to luck, genes and a healthy lifestyle.

Doctors leading the New England Centenarian Study — the largest study of 100-year-olds in the world — have worked with 1,500 people, including centenarians, their siblings, children and grandchildren. About 75 percent of the centenarians studied were functionally independent until age 95. Their children seemed to follow in their footsteps with significant delays in cardiovascular disease, diabetes and overall mortality.

In the United States, about 50,000 people, or one out of every 10,000, are 100 years of age or older, according to the 2000 U.S. Census. They are nearly 85 percent female and represent all ethnicities. Demographers expect the number to skyrocket throughout the 21st century. One census report estimated there will be about 800,000 centenarians by 2050.

Demographers estimate that as more and more Americans live to 100, more children will be able to know their great-grandparents.

A demographer at the University of California, Berkeley has estimated that by 2030, more than 70 percent of 8-year-olds will likely have a living great-grandparent, the New York Times reported Nov. 2. Some demographers refer to it as the great-grandparent boom.

The New England Centenarian Study has found that centenarians vary widely in years of education (no years to post-graduate), socioeconomic status (very poor to very rich), religion, ethnicity and patterns of diet (strictly vegetarian to extremely rich in saturated fats).

The commonalities included a healthy weight, a history of nonsmoking and an ability to handle stress better than most people.

Another study by researchers at the University of Chicago found women and men who were the first born in large families were two to three times more likely to make it to 100 than later-born children. Those raised in the rural West also had a better chance of living more than a century. The conclusions were culled from 75 million computerized genealogical records of centenarians born from 1875 to 1899.