Using information from a 2013 Gallup World Poll, the researchers found happiness patterns vary greatly by country. Respondents from high-income, English-speaking countries generally stuck to that U-curve.
However, those from Eastern Europe and Latin America showed a gradual reduction in happiness with age, and those in sub-Saharan Africa showed little change in happiness as they grew older.
Why is that? Well, researchers aren't exactly sure. One spoke with The Atlantic and offered a common-sense theory:
"In places with poor healthcare, no social safety net, or cultural disdain for the old, by contrast, the negatives of aging may outweigh the perks of 'emotional wisdom.'"
You can read the full study in the latest edition of The Lancet.
This video includes an image from marchasselbalch / CC BY NC ND 2.0.