You may have already seen this: Greg Mankiw directs us to this fascinating NYTimes article describing research linking education to longevity.
The answers, he and others say, have been a surprise. The one social factor that researchers agree is consistently linked to longer lives in every country where it has been studied is education. It is more important than race; it obliterates any effects of income.
As professor Mankiw notes, the author of the article does an excellent job in pointing out that researchers have been careful to try to entangle the causation-vs-correlation bugaboo that bothers so many of us. It appears that more education leads to longer lives.
A purely idle thought: I don't know this literature and maybe someone has explored this, but does it matter whether someone is formally educated? In other words, suppose Bill and Phil, identical twins, both graduate from high school and have exactly the same amount and quality of education. Suppose Bill goes off to college and eventually earns a PhD in Economics. Phil doesn't go to college, but gets the same education in the same amount of time by reading on his own. Can we expect Bill and Phil to have the same longevity?