Here is the gist: "People are least happy in their 40s and early 50s. They reach a nadir at a global average of 46"
It’s pretty easy to generalize a couple of models to show why some are happy later in life and why some aren’t.
As example, if one were to look at Maslow's hierarchy of needs one would find a hierarchy of behaviors and goals that are typical of many successful people. Successful people are the models upon which Maslow based most of his studies.
In contrast, if one were to look into Adler, one would find a pattern of behaviors that are typical of many who are unhappy. Not surprisingly, as that the types that Adler studied.
Accordingly, both point to a key detail to being happier at any time but particularly later in life, which is to spend a fair amount of time in comfortable and positive social settings. Nearly everyone gets a lot of self-worth by being in and contributing in a positive way to groups. And of course the types of social interaction one participates in, plays a huge role in one’s sense of self.
Of course, most are neither highly successful nor complete failures and due to this (and all things psychological) ymmv.