The Guardian:
Life gets better after 50: why age tends to work in favour of happiness
5 May 2018
Not all economists and psychologists agree. Economists Paul Frijters and Tony Beatton factored in the possibility that those who become happier in the studies are the same people who are more content when they start out. This can help them achieve greater career or relationship success, which leads to more happiness. Correcting for this effect, the U-shape disappears.
Related publications
The mystery of the U-shaped relationship between happiness and age , Paul Frijters, Tony Beatton, Journal of Economic Behavior & Organization, Elsevier, May 2012.