It hasn't always been 21 everywhere.
And I think there are still some minor differences about purchasing alcohol, from state to state.
But where it was really interesting was in states that had 18 as the legal drinking age several decades ago. In essence, that meant high school kids could legally buy booze.
"Old enough to get drafted, old enough to buy a beer" was one rationale in the early 1970s.
Anyone remember that?