Yeah, he's old enough to fight in a war, but he's not old enough to smoke cigarettes or drink alcohol.
I imagine that's very confusing to some people, but I've learned to accept it.
I've had 3 sons pass through 18 and I have another son who is 18 now.
My first two went through a lot of trouble in their teen years. My oldest didn't straighten out until after his first experience with adult court at 18. A plea in abeyance regarding a drug charge wasn't the same as escaping any legal consequences, but he was able to move on with his life eventually.
He was just a stupid 18 year-old kid.