A large Gallup poll has found that by almost any measure, people get happier as they get older, and researchers are not sure why.
This may be considered a more subjective and methodological correction, but Corrections considers such research to generally be invalid due to heterogeneity in responses by class. For example, wealthier people may simply respond to questions differently than poor people. Not because they are "happier," but due to some difference in type--rich people are likely different from poor people. Similarly, older people may respond to questions about happiness different from younger people.
There may be a saving grace for happiness research, exogeneous variation. For example, we might survey 1,000 individuals and discern their subjective happiness responses. We may then observe them a few months later--perhaps one has won the lottery or inherited some large sum of money. In this case, we can difference out his previous responses to his new responses, and attributed the difference to the large exogenous shock in wealth.
However, we can never get any such variation in age. We cannot take a young person and suddenly put them in an old person's body. The research, as Corrections sees it, is largely bound by the untenable structural assumption that old people's responses mean the same thing as young people's responses.