21 is an arbitrary age for drinking in the US, and having a group of people who certain amounts of rights but not all of them creates an implicit two tier level of adulthood. The laws are impossible to enforce; many places don't ask for ID, and those that do are circumvented by others.
Those who serve in the armed forces to defend the country are often as young as 18 years old. To say that they are not going to be allowed to drink alcohol is just plain silly. And college students under 21 years of age are known for getting their hands on alcohol even when it is illegal.
If an 18 year old can smoke, vote, live alone (without their parents), go to war, etc... then why can't they drink. We should definitely lower the legal drinking age in the U.S. to 18 for many reasons. One of them is so that these young adults can stop drinking illegally. Truth is many of them drink already and there is no need for them to go to jail for this.
The legal age of adulthood in the U.S. is eighteen. Those who are eighteen can register for active duty, vote, and smoke, own a home, and enter in contracts. If it is generally believed that one of sufficient maturity to make decisions such as these at the age of eighteen, then one should be mature enough to make decisions about alcohol consumption as well.
The drinking age should remain at 21 in the United States. While some teens are already drinking and certainly not obeying the law, it's still important to have it in place so that teen drinking doesn't become more common. Teen drivers are already the most dangerous of drivers on the road. Giving them permission to drink on top of that will make drunk driving more common, and the roads far more dangerous than they already are.