i have a 16 year old friend and we talk up to par on any issue; i usually forget he is much younger than me. also, my dad can throw quite a tantrum from time to time and i forget he is like three decades older than me...
i feel exactly like ed norton's character in fight club, when he says "i can't get married, i'm a 30 year old boy".... and i know i'll always be. what changes is, as i already said, that as years pass we get more information and so a bigger pool of data to make decisions with. because of this we could say that we get, maybe, more power. and as we all know, "with great power comes great responsibility" :)
so,
adulthood = time spent as a conscious entity gathering information + ability to apply said knowledge and learn from past experiences
some people have lived through a lot but they never learn.
some have not lived much but are very self-aware and thus mature rapidly.
what to do you think of girls? they seem to mature faster than us boys. maybe society expects them to and this creates a self fulfilling prophecy? i am wary of making sex distinctions because usually when i think about this distinctions i end up concluding that we are indeed just the same but playing different social roles.