Well , it isn ' t as simple as 21 is the age across the board . The federal government will withhold 10 % of each state ' s highway funding if they don ' t have some kind of 21 age limit . Typically this applies to unmarried individuals in a licensed public establishment without parental consent . However , many states have bizarre exceptions that allow underage persons to purchase with spousal consent , or consumption in private locations regardless of marital status . For example , California disallows purchase by someone under 21 without spousal consent , but consumption in a private location is legal . There is no internal possession law , so , there are no real MICs minor in consumption . There may be a 415 PC violation , which is disturbing the peace , but only if the minor is really fucked up . There is , however , a contributing to the delinquency of a minor charge if you are not the guardian but provide alcohol to an underage person . There is even more to it than that .