Again, I'm not disagreeing with you but just pointing out the other side, but:

A) The Constitution still applies. You still have freedom of speech under 18, but you still can't vote. Doesn't mean the Constitution doesn't apply to you just because you are under the age of something.

So,

B) Just because they are under 21 and can't buy a gun doesn't mean that being forced into the military meana you can't be taught how to properly use a gun under the correct supervision, and that you don't have the same rights as everyone else. Just because you are under 21 doesn't mean the Constitution doesn't apply to you. You still have the right to a gun, just not until you're 21 or in the military.

Same argument as drinking. Agree with it or not, that's what it is. Thats why drinking used to be allowed on military bases at 18. You could argue that drinking isn't a constitutional right, but in a technical way, its been shown to be a right by the repeal of the prohibition amendment.