After Prohibition, the age was usually 18 depending on the state, but then in the 1980s President Reagan created a highway subsidy program which basically bribed the states under the condition that the drinking age was raised to 21.
As a result, immaturity, irresponsibility, partying, DUI, and bar attendance increased by an additional 3 years, which continues to this day.
When you compare America to European countries which don't have a legal drinking age, you realize that this behavior is irreversible in the short term and thus no politician will ever lower the drinking age back to 18. Except the Libertarian Party or maybe Ron Paul.
Hey, I'm 21, I can legally get Drunk! Let's Party!
2. The day of the month when apocalypses are known to occur.
2. May 21, 2011, October 21, 2011, December 21, 2012. Need I say more?