It was 21 after prohibition and then lowered in some states to 18 or 19 in the 70's. This was right around when the voting age was lowered to 18 as well, people thought the same "if you can vote, you can drink". The idea didn't work too well. Studies showed huge increases in teenage DUI's and accidents and the age was raised again in most states. The the federal government stepped in and required it to be raised up to 21 all over or the state would lose certain funding.