In fact, they found the opposite; of those who were surveyed, the people in their 20s were the most stressed out and depressed. The folks in their 90s, on the other hand, were the most content, even with things like loss of loved ones, a decline in health, and the aforementioned joint pain and slower metabolism to worry about.
Maybe the best news of all is that people seem to experience a steady increase in overall happiness as they age; researchers didn't find a drop in middle aged people's sense of well-being. So maybe buying that Corvette really DOES help! Whatever the individual factors, the older people got, the better their mood, outlook on life, and handle on stress all were. The grumpy old man trope is more fiction than fact.