When I was 18 the States and some local jurisdictions determined the drinking age, and in some it was in fact 18. For instance my home State of New Jersey was 21, but right across the border in New York State you could drink legally at 18, while even closer to me in New York City it was 21.
Hence lots of us Jersey teens drove into New York State to drink. But many also got killed returning drunk on winding back roads, and the rate of automotive accidents & fatalities was blamed on teen drinking. States like New Jersey lobbied for a national standard of 21, to prevent the cross-border drinking & driving by teens. (Apparently universally lowering to age to 18 wasn't considered the better option)
This at the same time that the Federal voting age was being lowered to 18, partly a result of the unpopular Vietnam War. The argument was that young men could be drafted to fight & die for their country, but they weren't old enough to vote about it. But on the other hand now they couldn't drink to it. A bit schizophrenic all around.
And BTW, many States, especially in the South where I was stationed, had "dry" counties where no liquor could be sold at all. It kept a lot of us soldiers in the on-post clubs, the only places we could drink (at 18 before the Federal law changed). Even today there are dry counties, but the work-around is the bar calls itself a private club, and you have to buy a membership to enter. Then you can drink. Typical US lunacy, a mix of religion and right-wing politics producing illogical results.
