Slate Magazine, in The Midlife Happiness Crisis, reports on "a new study by economists David Blanchflower of Dartmouth and Andrew Oswald of Warwick. They document how happiness evolves as people age. While income and wealth tend to rise steadily over the life cycle, peaking around retirement, happiness follows a U-shaped age pattern... Happiness starts off relatively high in early adulthood, then falls, bottoming out on average around age 45, and then rises after that year and on into old age."
As a relatively new retiree, I'd like to second that notion. Retirement is a wonderfully happy time, after you've done a bit of adjustment. First, you have to get over the guilt you feel when everyone else seems to be zipping off to jobs and stress in the morning. Then you just have to find a way to use the lovely gift of time that you've been given, in a nice balance of service to others and giving yourself permission to do all the things you've been wanting to do--and hoping for the physical capability to do all those things.