I agree that the age should go down incrementally, in case things go terribly wrong (unlikely) when the age is lowered from 21 to 20. 19 might be good, because it still restricts high schoolers from drinking, but allows almost everyone in college to, and there's so little to stop them anyway.
I think the best course of action is to allow drinking in bars at 18, and allow purchase at liquor stores at 21. Hopefully 18-year-olds would pick up better drinking habits at bars, where the expense of booze would limit quantity, rather than when they buy a cube of Icehouse and try to drink it as fast as possible to avoid being busted. I generally observe much more responsible drinking at bars vs. parties.