Americans' lifespan shorter than previously thought, study finds

A group of researchers has developed a more accurate way of calculating mortality rates of the oldest Americans, which might lead to more realistic cost projections for Social Security and Medicare.

Demographers have traditionally believed that the mortality rate of Americans flattens out above age 80, and the U.S. Census Bureau predicted that the 2010 census would find 114,000 people age 100 or older. However, the census revealed only 53,364 centenarians.

Experts at the University of Chicago say that's because the rate of mortality growth with age of the oldest Americans is the same as that for those who are younger. A person's risk of death in a given year after age 30 doubles every eight years of age, which is known as the Gompertz Law.

The study was published in the current issue of the North American Actuarial Journal.