1979 Mistake

You are right and wrong. In the movie he ordered a bourbon at the bar, not a beer, which is why the bartender asked if he was 21. The national drinking age for beer was 18 and 21 for liquor, until 1984.

In 1984, the federal government started increasing the drinking age over a 3 year period from 18 to 21 for beer to match the 21 limit for liquor. Some states like louisiana and texas were going to ignore the change since they felt it was an encroachment on states rights, which it was, but the federal government declared any states not complying would lose their federal highway funds. thus, the feds hijacked states rights to get their way on this despite it being unconstitutional, unfortunately all the states were too scared to fight it in federal court. I remember all of this well because I graduated high school and was a freshman in college in 1984. I was legal to drink beer when I turned 18 in March of 1984, then in late 1984 that changed to 19, so I was temporarily illegal until I turned 19, making me legal again. But then the age went to 20, so I was illegal again. Finally i made it to 21 and all bets were off. It was such a royal pain and most people still disagree and there is a movement to lower the drinking age for beer back to 18, which they should. You can vote at 18, die for your country, and be tried in court as an adult, but you can't buy a beer. what a joke!

but to get back to your question, like i said above, re-watch the movie, he ordered a bourbon, which is liquor. you had to be 21, then just as now, to drink liquor by law anywhere in america. you must of thought he ordered a beer.