21, even though I haven't got that far yet its just so when I visit my American family I can go to bars ect.
Anything else I'm happy with the age I am now (19). That way I can travel most of the world to the full extent and I'm not overly young.
16 would suck in my opinion, even though you have no real responsibility your still quite bound to home and can't really go very far totally on your own.