Where are people able to drink before the age of 21?

Legally, no you cannot. There is a federal law against it. You might be able to buy it from people illegally and you can drink it in secret. If you are planning to be a loser in life by any chance, you can go to random partys and get as drunk as you would like. Don't expect to get away when the cops arrive though.

In conclusion, don't drink underage.