Personal experience...
Me (24yo at the time) and 6 jarhead buddies (18 to 20 in age) from Camp Pendleton on weekend liberty sipping beer on the beach in Cali in 1989. Cops showed up, the youngsters all got fined for underage drinking and I got cited with Contributing to the Delinquency of Minors.
AND the cops made us pour out nearly a case of beer on the beach.
So yes, said 21yo is culpable under the law - even if s/he is a US Marine.