I think this graph misses off a group struggling with ill health and pain in later life that say things like come on death, what are you waiting for. They are done, stuck in a long decline.

Pretty accurate graph for me so far. I would say "it gets better" is more true. Childhood kind of sucks and early adulthood is poverty and stress. 30s the nadir maybe, not much worse than childhood. But from my 40s things have improved, and so has my mental health and happiness. Like I got more competent at more of my life, finally.

...huh. It centers right on the working years, doesn't it? Plummets when you would enter the workforce, and rebounds right around retirement

That makes sense for the average person maybe. Unfortunately, I am not the average person and I will almost certainly be cripplingly even more sad in old age. I have strong reasons to believe this, largely due to having missed out on life more or less by sheltering myself from it in my youth. And as I age I'll continue to know this and it'll become more and more cemented and bitter.

Though I will say, this also might be influenced by the fact that they merely surveyed a slice of people, and older people might have been happier in 2012 (and continue to be so) because they have more money than previous generations will have in their own old age.