In my opinion, if you can die for your country at 18 you should be able to have a beer.
At my age, I can have a wine or a beer in Germany with adult supervision. In fact, I had wine with my dinner earlier.
If we keep treating 21 as such a big deal those under that age are more inclined to drink as minors, since they see it as if they're doing something so huge and scandalous.
It's like if you ban smoking, people will just find illegal ways to obtain cigarettes.