the 21 drinking age did not change the base club drinking age to 21. beer and wine have been available to the underage service members. they were generally not harassed for drinking on military beaches, at military events, etc.
"However, in the 1980s, anti-alcohol groups such as such as MADD (Mothers Against Drunken Drivers) lobbied Congress to prevent this. “Federal law (United States Code) requires military installation commanders to adopt the same drinking age as the state in which the military base is located. The only exception to this rule is if the base is located within 50 miles of Canada or Mexico, or a state with a lower drinking age, the installation commander may adopt the lower drinking age for military personnel on base.” http://www2.potsdam.edu/hansondj/InTheNews/UnderageDrinking/1106156025.html