Live to be over 100? Why not?

As he hits 90 this month, Supreme Court Justice John Paul Stevens often still plays a game of tennis before a day on the bench. And only now is the respected jurist planning to leave the court.

It is a measure of society's changing ideas of "old age" that Justice Stevens's advanced years and his vibrant mental capabilities have drawn little media attention.

Indeed, as a story in The Christian Science Monitor about the high number of centenarians points out (click here), life span may be as much a matter of attitude as anything else. Who can say when it's time to "retire"? When Bismarck picked 65 as the retirement age for Germany's 1889 pension program, there was little regard for progress in human thinking about aging.

Today's researchers are scrambling to explain the rapid rise in Americans who are 100 or older. The numbers of centenarians has increased 35 percent from 1990 to 2000, and may have risen 50 percent in the past decade. This century, the growth rate may be 60 percent per decade. One study predicts that half of all children born since 2000 could live to 104.

While research on aging often focuses on physical lifestyle, family background, and medical care, more attention is being paid to how people mentally work at battling notions of decline. How well do they negotiate life's troubles? What role does humor play? How outgoing are they? (One way to know seniors want to keep alert: They keep balancing their own checkbooks.)

In general, those who reject retirement and a later life of leisure in favor of work or volunteering seem to have a leg up in adding bonus years. Giving to others and to future generations is also a gift of life to one's self.

The rising numbers of centenarians should be a signal to "young 'uns" in their 70s and 80s that they have energy to spare – and stereotypes about health to break.