> If you are going to use the inherit argumenT, then why not 14 or 9?
It's a good question. It doesn't seem correct, but I can't find any argument for having it at 16 or 18 or 21 that doesn't also apply for having it at 14 or 9 either. But having it at 25 or 30 based on development smacks of disenfranchisement and eg jim crow literacy laws.
It's not a question I think we can get a satisfying answer for through a scientific-medical model. An 18 year old is still developing, but is also still capable of making informed decisions. So to a lesser extent, is a 16 year old, and to an even lesser extent a 9 year old. And adults who for any reason are less intellectually sophisticated still have the right to vote, which again is a matter of societal values not medicine.
Looking at cultures through history, the timing of transition from childhood to adulthood, the rituals that mark it, the responsibilities that accrue with it, that's really what we're talking about here. The age varies quite a bit across time and place (and different groups may have different reasons for placing it where they have settled on, as do we) but 14-20 probably catches the majority of when most people would consider this change to have taken place. And 16-18 may be pretty much the median? I don't want to assert it too strongly but probably anything within these ranges is "fine" with certain tradeoffs one way or another.