Things like smoking and excessive drinking for the sake of showing that you're Cool and Rebellious for doing the exact things that the adults say you shouldn't do, for example. It's easy to see why that kind of behavior might emerge in an environment where other kids your age are your ingroup that you want to impress, and adults are the outgroup that you can attack in order to distinguish yourself. But if adults were actually the ingroup you were trying to impress, it seems like people would be more likely to try to impress them by actually acting more mature, and that "maturity is high status" would carry over even to the more limited interactions they had with folks their own age.