Until very recently, most of the increase has been due to better survival in childhood and youth, rather than the increased prolongation of middle age into old age.

ECONOMIST: Why women live so long

Given that the period opening up between midlife and anything resembling old age is growing and could be as long as the middle years in duration it's worth experimenting with new roles.

WSJ: The Experts: How to Avoid Retirement Boredom

Unlike developing countries, which age from the bottom up because of a falling birth rate, rich countries age from the middle, adding old people with fewer workers to support them, and even fewer children to join the workforce.

ECONOMIST: Economics focus

Since 1995, Dr. Ryff and her Wisconsin team have been studying some 7, 000 individuals and examining factors that influence health and well-being from middle age through old age in a study called MIDUS, or the Mid-Life in the U.S. National Study of Americans, funded by the National Institute on Aging.

WSJ: University of Wisconsin Study Finds Eudaimonic Happiness Lessens the 'Bite' of Risk Factors for Disease