... [P]sychological well-being moves along a U-shaped curve as we age. ... Happiness among American men and women reaches its estimated minimum at approximately ages 49 and 45 respectively. Among European men and women, life satisfaction levels are at their minimum at ages 44 at 43 respectively.
That's from this NBER commentary on the work of David G. Blanchflower and Andrew Oswald (which I haven't read). Here are some of the plausible reasons for why happiness levels pick up after reaching the minimum:
... [F]irst, ... individuals learn to adapt to their strengths and weaknesses, and in mid-life quell the unfeasible aspirations of their youth. Second, ... cheerful people live systematically longer than those who are miserable, and that the U-shape somehow traces out, in part, a selection effect. Third, ... a kind of comparison process is occurring - for example, I may have seen school-friends die and as a result eventually come to value my blessings during my remaining years.
What about this finding (articulated in a Daniel Gilbert column)?
Studies reveal that most married couples start out happy and then become progressively less satisfied over the course of their lives, becoming especially disconsolate when their children are in diapers and in adolescence, and returning to their initial levels of happiness only after their children have had the decency to grow up and go away. When the popular press invented a malady called "empty-nest syndrome," it failed to mention that its primary symptom is a marked increase in smiling.