All fair points, but I never defined adulthood, only disagreed with the parent which stated "the cut-off is not biological, it's social". Of course there are social factors as well as biological. I'm sure I'm not qualified to form such a definition, though if I tried it would have to include many factors.

I, for example, moved out of my parents' house at 18, but I wouldn't say I became an 'adult' until at least my early 30's. Even now I feel like a kid most of the time, in spite of my many responsibilities - even with a kid of my own!

I don't think we ever finish maturing, and I think we never stop learning, even though our 'peak' mental ability may come and go.