When I was in college, the drinking age moved up one year at a time, from the time I was 18 to 21, so I was lucky. The theory was that they didn't mind 18-year-olds drinking, but that if the drinking age was 18, that meant in reality that kids three years younger were drinking, so if you raised the age to 21, in that new math, 18-year-olds would be drinking. To my peers, that was WTF?
States, which control the drinking age, caved into the Federal government, which threatened to take away Federal highway funds from states that refused to raise it.