The happiest time of my life?

I heard a comment on my favorite TV health minute this morning that I found interesting but not too surprising.  People get happier, more content, as they get older. Conversely, people tend to be most discontent in their 40's.  Well, no big surprise there - if you're a parent your children are probably in or approaching their teens with all the stresses that can lead to: dating, driving, part-time jobs, finding money for college, the list goes on when you're in your 40's.  Whereas, when you are rapidly approaching 70 (as I am) your children are all grown up, probably in their 40's, and their children are their concern, while you get all the fun. Why does it take research to figure this out?