The only reason for 18 really is due to the Vietnam era and the voting rights of those being drafted. Given the draft is not longer in place unless in extreme circumstances and even then is only males (so much for equality), it means the magic age of 18 is irrelevant. Human beings start to get near the mental development after 20 years of age, and during 20-25 they finish developing their brain. So for voting, adulthood etc 21 is actually closer to maturity than 18. How many 18 year old do you actually know are capable of making an educated guess when voting. I tell you, anyone in Honor and AP classes should get an exemption, since its all a matter of knowledge. More knowledge being processed the more your brain develops.
So to put it simply, 21-23 should be the adult age, however if you were an honor or ap student in highschool you may choose to have the age be 18.
First of all, there are several different definitions of what it means to reach "adulthood". It could be as early as 13-15, when adolescents become sexually mature. It could be as late as 25-28, when the adolescent brain finishes developing into a more adult brain.
Second, every individual has a unique body and therefore, every single individual would have a unique time upon which they become an adult. Keeping track of such minute developmental detail for every citizen would take way more effort than it is worth.
Having an easy, universal cut-off works just fine. It's arbitrary, so it doesn't work all the time, but it works better than any other realistic solution we have come up with so far. Deal with it.