A new (2017) research project from the US National Bureau of Economic Research looked at the psychological well-being of 1.3 million randomly sampled people (age 20 to age 90) across 51 countries to determine if anecdotal reports of widespread 'Midlife Crisis' are based in fact.
Finding in general that a U-shaped graph frequently describes well-being over lifespan - i.e. happy in early and later life with a dip in the middle.
The paper applies each to large cross-sections and compares the patterns of life-satisfaction and happiness. Using the first method, there is evidence of a midlife low in five of the seven data sets. Using the second method, all seven data sets produce evidence consistent with a midlife low. The scientific explanation for the approximate U-shape currently remains unknown."
Source : Do Humans Suffer a Psychological Low in Midlife? Two Approaches (With and Without Controls) in Seven Data Sets. IZA Institute of Labor Economics.