Replying to Question about serving alcohol

so here's the deal.. i am 20 and work at a bar/grill. if a friend of mine comes in (who is of age) and i serve him, and i buy him a drink (so let's say he buys two millers at $3 a piece and i only ask him for 3 bucks - i'd cover the rest) is essentially what i did illegal? because i am underage and technically just bought alcohol for someone? or doesn't it really matter because i could just cover it as i just put it through my tips since i am actually working and handling the alcohol.

honestly it isn't all that important, i was just curious to see if anyone knew the answer to this, cuz i don't.