Economists David Blanchflower of Dartmouth and Andrew Oswald find that happiness follows a U-shaped age pattern.
Analyzing data from these surveys, Blanchflower and Oswald found that 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.via Slate
In this study (as in others), people are happier than their poorer counterparts if they have more income. How does the effect of income on happiness compare with the age effect? 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. For Europeans, the age-based happiness rise that's equivalent to the effect of doubling income occurs between ages 35 and 70.
The authors also find that over the last century, Americans, both men and women, have gotten steadily—and hugely—less happy. The difference in happiness of men between men of my generation, born in the 1960s, and my father's generation, born in the 1920s, is the same as the effect of a tenfold difference in income. In other words, if my father had little money compared to his contemporaries and I have lots of money compared to mine, I can still expect to be less happy. Here, curiously, the European pattern diverges. Happiness falls for the birth years from 1900 to about 1950, and generations born on the continent since World War II have gotten successively happier.