An 18 year old in the Army can walk into any bar and order a drink?
No. Looking it up, I find , though it was always that way, and only on base, the law has changed:
http://usmilitary.about.com/od/justicelawlegislation/a/drinkingage.htm
“An 18 year old in the Army can walk into any bar and order a drink?”
NO but he can drink at the EM Club or purchase at the commissary.