Ezra Klein supports lowering the legal drinking age to 18, with one catch:

21 is, of course, a bizarre marker. Demanding that kids refrain from drinking for three years after they become legal adults and, in most cases, leave their parent's supervision, is a bit odd. "Welcome to adulthood, except when it comes to beverage choice!" But this could point the way towards a grand new education policy scheme: Drinking age is 18...if you attain a college-worthy GPA. Otherwise, 21. Implement that and you'll blow those other, way lamer, educational attainment proposals out of the water.

Genius.