Acting like an adult is not the same as being an adult.
Playing the part is still childish, no matter how serious one claims to be.
Caring about others may be more dysfunctional than watching out for one's own.
Doing what you want may serve others more than trying to be useful in a way that others may appreciate.
Instead of people wasting their time and energy trying to guess and second-guess what other people want, we would all be better served by adults acting like "children" in that they look out for their own interests first, without violating the rights of others to care for their own.