Economist: ... People, studies show, behave differently at different ages. Older people have fewer rows and come up with better solutions to conflict. They are better at controlling their emotions, better at accepting misfortune and less prone to anger. In one study, for instance, subjects were asked to listen to recordings of people supposedly saying disparaging things about them. Older and younger people were similarly saddened, but older people less angry and less inclined to pass judgment, taking the view, as one put it, that “you can’t please all the people all the time.”
There are various theories as to why this might be so. Laura Carstensen, professor of psychology at Stanford University, talks of “the uniquely human ability to recognise our own mortality and monitor our own time horizons”. Because the old know they are closer to death, she argues, they grow better at living for the present. They come to focus on things that matter now—such as feelings—and less on long-term goals. “When young people look at older people, they think how terrifying it must be to be nearing the end of your life. But older people know what matters most.” For instance, she says, “young people will go to cocktail parties because they might meet somebody who will be useful to them in the future, even though nobody I know actually likes going to cocktail parties.” ...
I actually like cocktail parties :-) Elsewhere in the article they note extroverts tend to be happier than introverts. What about hypomanics?
Note Added: Andrew Gelman and commenters point out that the data is not as clean as described in the Economist article. See discussion here.