I always find the amazingly backwards American attitude to alcohol completely astounding.
So you can get the death penalty, drive, run around with guns, get married, go to Iraq and die for Uncle Sam, smoke, be sent to adult prison, pay taxes... but you can't have a beer? Give me strength. If you're that backwards and anal about it it's no wonder kids are getting hammered whilst still underage.
In the UK the legal age for drinking in private is five years old, although if you got your children plastered on a daily basis you'll have social services all over you like a rash. The aged eighteen restriction here involves the actual purchase of alcohol but it's too easy to get it underage.
Yes, it should be lowered to 18 in the US. It's best to introduce you to alcohol as early as possible so that you have some decent education and direct life experience of it instead of being like kids in a playground at age 21.