Being an adult essentially means being mature. Maturity is extremely hard to define or discern. Given a choice between the 26-year-old student who's going directly from Mum's lap into Dad's law firm without ever having had to earn a penny himself and the 23-year-old who learned the masonry trade at 16 and now already has a broken back and is struggling to get a start-up enterprise going (I know one of those), it's clear -- the ex-mason clearly is more of an adult.
Is he really? (Now I'm drifting into fiction.) The cute little ex-mason has, for some eight years now, been treating people of both sexes like dirt -- picking up and dumping girlfriends all over the place, all the time; deceiving his friends in any way imaginable. He clearly is responsible for himself, since he lives on the money he earns. But other than that, he's as egoistic as a little child.
The student, on the other hand, may look like an arrogant, useless git. But he knows what his duties are. He votes; he abides by the law; he'll always pay his taxes; he will pay for his ex-wife and children, should he ever have to divorce. He knows a fair bit of the inner workings of our society. And even if he's never had a supporting role in it -- when a crisis comes up where his help is needed, he'll try to help. Probably, he'll just go down, his flag gallantly waving, because his skills are rather limited. But the ex-mason-turned-entrepreneur will probably just run away if something looms at the horizon.
What does this tell us? Not damn much. Maturity is a way of thinking and not a material way of living.