Actualy in the state of Texas there is no drinking age in 2 situations. First if the under age persons parents take them out drinking, Second is if the under age persons spouse takes them out drinking.
It was interesting when I was stationed in SanAntonio, there was a 22 year old guy with a 17 year old wife and we all use to go out drinking.
Also Fort Bliss Texas has a leagle drinking age of 18 as long as you dont leave the post. The reason we were given is that there so close to Mexico that its safer to let them drink on post then to have them go to Mexico to drink.