That explains it: I'm 49

Over at Slate, Joel Waldfogel writes:

While income and wealth tend to rise steadily over the life cycle, peaking around retirement, happiness follows a U-shaped age pattern....for both men and women in the United States and throughout Europe, happiness starts off relatively high in early adulthood, then falls, bottoming out on average around age 45, and then rises after that year and on into old age....In the United States, the steady decline in happiness from age 16 to age 45 has an effect that's larger than a 50 percent reduction in income—that is, happiness varies more as people get older than it does if you compare significantly richer people to poorer ones. And, equivalently, the 15-year upswing in happiness that follows age 45 is stronger than the upswing that tracks doubling of income.