In an ideal world, you would be right, in fact, it'd be nice is no one ever had to fight in war; however, the draft age (or even the enrollment age, to a certain extent) is based on necessity. In the case of the draft, everyone is needed. Do you think the Europeans wanted to draft 15-16 year olds during WWI & WWII? Of course not, but sometimes you don't have any other choice.I think they need to up the military enrollment/draft age to 21 and for anyone who enlists your first three years of college are free (to a non-private college). After your third year you can opt to go into the military, or take your final year and do your service after receiving your degree.
Hopefully, we'll never need to impose the draft again, but if the need arises, we know where to find everyone.
There is no necessity for alcohol, on the other hand, so the two age limits cannot be accurately compared.