A glance at the latest charts on life expectancy--now versus then--seems to affirm a popular idea that Americans are speeding toward life beyond age 100.

In "Chartbook on Trends in the Health of Americans 2006," published last month by the National Center for Health Statistics, are upward trajectories dating to 1900: Life expectancy at birth climbed in that time to 75 years from 48 for males and to 80 years from 51 for females.
But don't assume your last will and testament can wait. Demographers are tangled in a debate that casts doubt on the likelihood of old people growing much older than ever before.

"The distortion in [life expectancy] numbers," says researcher John Bongaarts, vice president of the New York-based Population Council, "is largely due to steep reductions in infant and childhood mortality."

The spiraling of life expectancy has spurred an industry of hopeful books such as "Healthy at 100," seminars such as "The 120 Club: Living the Good Life for 120 Years," and Web sites with "life-span calculators."

However, many experts believe humans are approaching a biological "ceiling" for old age; some even expect the longevity curves to start heading south.

Consider: In 1850 a white American man lucky enough to reach 60 could expect to live 16 more years on average -- to age 76, according to federal health estimates. In 2004 the law of averages suggested he could bank on living a few weeks short of 81.

The upshot? Despite trillions of dollars spent on making Americans healthier, white male seniors apparently have gained only five years since the era of cholera epidemics. White women reaching 60 chalked up seven extra years.

"Even if you add five or six years of life expectancy for people who reach their 60s, that's not trivial," says public health professor S. Jay Olshansky of the University of Illinois-Chicago. "But, certainly, to extrapolate overall life expectancy and say that we or our kids are going to live to 100? That's ridiculous."

Olshansky is among the public-health prognosticators who foresee U.S. life expectancy reversing course "in a couple of decades," as higher rates of obesity take their toll.

Still, centenarians are growing in number. The Census Bureau estimates 79,600 Americans aged 100 and older, compared to fewer than 20,000 a quarter-century ago.