Want to be happier when you're old? Get a job.

In recent years, researchers have made a lot of progress in understanding how happiness changes with age. The relationship is now widely believed to be a U-curve: The level of well-being drops until one's late 40s and then starts to go up again. An important 2010 study by Arthur Stone of Stony Brook University and his collaborators showed that in the United States, people are happier at 80 than at 20. While the level of sadness remains relatively constant throughout a person's life, with a slight bump in middle age, older people worry a lot less, experience much less stress and are less frequently angry.

More recent studies show that the U-curve holds throughout the world, with a few exceptions. There are, however, important variations: The age where the curve bottoms out differs across countries, and the subsequent increase in subjective well-being isn't equally steep. Carol Graham of the Brookings Institution talked about it in a recent podcast. She plotted the curve for various countries based on data from the Gallup World Poll and found that the low point was at 44 in the U.S., 48 in Latin America and 50 in Russia. In middle age "people are stressed, people often have the double burdens of teenage kids and older parents," Graham explains. "There is also the effect of learning who you're going to be when you grow up. Expectations align with reality."