It used to be 16 to drive, 18 to drink, and 21 to vote. But then the 1960s and 1970s started to happen with the voting age being reduced to 18 in 1971 by constitutional amendment, and then Mothers Against Drunk Driving lobbied the federal government because drunk driving deaths were rapidly increasing among 18 to 21 year olds (actually due to Baby Boomers coming of age in the 1960s and 1970s) and then the federal government pressured states by threatening to cut their access to federal highway funding if they didn't change the drinking age to 21, leading to the complete flip-flop of the voting and drinking ages within a decade.