I know this is a joke, but I really dislike these negative characterizations of adulthood, especially because they seem to just suggest that we should be complacent with this kind of dissatisfaction and discomfort as if it's a intrinsic part of aging. I mean sure, maybe by the time you're 70 a lot of this is unavoidable, but that's far past being an adult. I often hear young people half-jokingly say that they may as well die by the time they turn 30. Interestingly, the same 30 to 50-year-olds who love to complain about teenagers dreading getting older are the same people who make them dread it by complaining about it. None of this stuff is really necessary to getting older, and again, I do understand that this is partially a joke, but I don't know...
This isn't being an adult. This is just taking bad care of yourself, physically and mentally. If you've always taken bad care of yourself, then it'll start to pile up by the time you're an adult. If you've always taken good care of yourself, that'll also start to pile up by the time you're an adult.