It was 18, for a while. High-school seniors fucked it up. Seriously. They were buying alcohol and giving it to their minor friends.

Yeah, I know it still isn't much of a barrier. But then, the news had a constant drumbeat of 16-17's that died or fucked themselves up on alcohol their buddy bought for them.

They raised it to 19 in some states. Other states limited people under 21 to 3.2% beer (which tastes horrid). Ultimately, the federal government said: you have to raise it to 21, or we'll cut off your federal highway funds.

They imposed a nationwide 55 mph speed limit the same way.

I turned 18 during that period, but was 21 by the time it was forced on the states.