I've always found it exceedingly bizarre that in the US, one can enlist in the military and be prepared to kill or be killed at age 17, while even at 20, it's illegal to purchase a non-alcoholic beer. I think America is very strange.

I'm a little unclear how that's different from any of the other things US federal, state, and local law allows minor children to do with their parents' consent. In any case under federal law, the lowest age limit for enlistment is 17 with parents' consent.

What do you mean by "underaged"?

17 years old or less, I presume

The minimum enlistment age is 17 with parental consent. Underage would be 16 or younger.