It is 21 in the U. S. everywhere. Some states used to have lower minimum drinking ages. The lobbying group, Mothers Against Drunk Driving, successfully lobbied to have the Federal Government withhold highway funds from any state with a minimum drinking age lower than 21.
The reason that hosts in the U.S. are focused on minors drinking alcohol is that there are civil and criminal penalties for giving or selling alcohol to a minor and for supplying a place for a minor to drink alcohol.
A minor’s parent is legally allowed to give the minor alcohol. We recently had two French guests, mother and 19 year old son. The mother asked if it would okay for the son to have a beer. I told her that as long as she was the one to hand the beer to her son it would be fine.