The research in Social Science and Medicine finds depression is most common among men and women in their forties. The dip in mental health seems to come on slowly.
The reason why middle age is a universally vulnerable time is unclear. Researchers say one possibility may be that people realize they won't achieve many of their aspirations at middle age.
The good news: many people start emerging from this low period in their 50s, and if you make it to 70 and are physically fit, you're likely to be as happy as 20-year-old.