Is it? Not in every state. Let us think about the actual code from 1984.
http://www.law.cornell.edu/uscode/uscode23/usc_sec_23_00000158----000-.html
As of January 1, 2007, only 14 states and the District of Columbia
ban underage consumption outright, 19 states do not specifically ban underage consumption, and an additional 27 states have family member and/or location exceptions to their underage consumption laws.
http://t in y url.com/anorz (remove the spaces)
So depending on the state, in this scenario if a teen had drunk a beer in a private home, there would be no law broken.