Studies based on the Gallup-Healthways Well-Being Index of the US are starting to trickle out. Researchers have used this data to replicated a well-known finding: global measures of well-being are U-shaped, peaking when you are young and old, with the inflection point at around age 54. Hedonic measures of negative affect (such as stress and worry), on the other hand, consistently decrease as we age.

Stone et al. A snapshot of the age distribution of psychological well-being in the United States. Proceedings of the National Academy of Sciences (2010) vol. 107 (22) pp. 9985-9990 doi: 10.1073/pnas.1003744107