I have one question regarding underage drinking in the US:
Is drinking when you are underage a crime? Or is it only a crime for the one who bought you the alcohol?
Not necessarily. The law varies from state to state. All states enforce the age of 21 years as a minimum age for purchasing or public possession of alcoholic beverages, although some states do not ban the consumption of alcohol by those under the age of 21 in a private setting, while others do.Both the drinker and the purchaser gets in trouble.
Plus, it can be dependent upon other factors as well. For example, in every state anyone under 21 caught with a BAC above 0 while driving will be arrested.
Also, if your caught purchasing alcohol for anyone under 21, you may be held responsible for their actions while under the influence.