For those who contemplate retirement as decades filled with leisure and relaxation, The Longevity Project serves as a warning. As Friedman says, "fun can be overrated" and stress can be unfairly maligned. Many study participants who lived vigorously into old age had highly stressful jobs. Physicist Norris Bradbury, who died at age 88, succeeded J. Robert Oppenheimer as director of the Los Alamos National Laboratory, overseeing the transition of the U.S. atomic weapons research lab from World War II into the Cold War.

Friedman and Martin say it's the kind of stress that matters. The bright boys selected for the study who ended up having low-status jobs--streetcar conductor, baker, porter--and whose careers did not match their early promise were far more likely to die before age 60 than their higher status counterparts. Success, even in challenging jobs with demanding hours and responsibility, is a tonic. (Ever notice that orchestra conductors and dictators tend to go on forever?)

Working late into life isn't just good for you, it's also good for the economy. During the recession, I recall some liberal representatives arguing that we should increase employment by inducing older workers off payrolls with a new super-low Social Security retirement age. This plan misunderstood that the point of an economy is to maximize production, not to maximize 20something employment rates by any means necessary, even if it requires pushing exceptionally competent older workers out of the economy to clear space for the next generations.