The good: It turns out that working longer is actually good for you, as studies show that those who work longer tend to live longer, too. "Financial consideration aside, the "golden years of retirement" are less personally satisfying than productive employment," Mislinski writes. She points out that this trend started in one of the best economies in recent memory, the late 1990s, when retirement funds were plump.

The bad: There should be no mistaking, however, that some of this shift is the result of growing retirement insecurity, too. Two-thirds of working households aged 55 to 64 retirement savings equal to less than their yearly income, according to the the National Institute for Retirement Security.

The ugly: These data also highlight the bifurcation of the American economy. While it's great that some older Americans are taking advantage of work that is fulfilling and well paying, it's happening at the same time that a shrinking share of prime-age workers are doing the same thing. It's yet another example of how the modern economy is generous to some (usually the affluent and well-educated), while leaving others behind.