Back when I was 18, it was legal for me to drink, hard liquor, when I truned 21 they made it legal for 19 year olds to drink beer. A halfway reversal of the law... Now it's 21 for any alcohol. Somewhere in there they made the voting age 18, the same age at which you could join (or be drafted) into the armed forces (without parental permission, IIRC).
Back then, If you were in the military, you could marry and drink, no matter your age. Inconsistencies don't work. Inconsistent laws work less...
If you want to make the age of majority 21, fine. But that should then be the age at which you can join the armed forces, and vote.
I agree that 18- 21 year olds are not yet as mature as they could be... I agree that their brains are not at their maximum growth.
But, if you must register for selective service at 18 (and you must, still), you should be granted ALL the rights of majority.
I would argue that most of our 18 year olds are not as educated as we were 30 some years ago... I would argue that many of them are not nearly as mature as we were at 18. But this is not the place for such arguments.
If it is okay for us to send 18 year olds to war (by their own choice), and we are to assume they KNOW what it means to swear the oath to protect the Constitution of the United States from enemies foreign and domestic; they must then be wise enough to know what it means to defend ones self in the gravest extreme, whether they choose to do either, or not.