At the age of 18, American citizens can, among other things, drive, own property, vote, get married, and even join the armed forces. They cannot, however, legally drink alcohol.
The argument for the current legal drinking age is that a person's brain is not fully developed and matured until they are 21. However, if a person is considered, by the government, mature enough to promise themself to another for the rest of his life, mature enough to have an opinion and voice on the next president, mature enough to risk their life for their country and possibly take the lives of others in the process, then that person is mature enough to drink.