The attitudes towards alcohol in the USA are quite bizarre to most of the rest of the planet but we didn't have prohibition.
When I went to the USA with the British Army, I found that although I was old enough to be an ally with a rifle, I was not old enough to have a beer at 20! I was old enough to go in harms way but not old enough for Budweiser! Your troop transport aircraft was supposed to be dry. I have heard that your naval vessels are dry.
Off topic, but one of the best times I ever had was drinking with some Welsh soldiers on leave from the British Army, in an Irish pub on the bank of the river in Salzburg, Austria on New Years Eve. Don't know if it's covered in basic training or what, but you guys sure know how to have a good time drinking. Also, we did you a favor on the Budweiser.