Getting older is not a recipe for crotchetiness. Although those two cranky Muppets will always be up in their balcony, Americans in general don't become less happy with age. If anything, they get happier.
The trajectory of people's happiness over a lifetime is tricky to study, because in a given year you're capturing not only your subject's age but also the current events. You need to follow a large group of people over many years, and you need them to be all different ages when the study starts.
