Well it depends on where you live but there are various answers here:
1) Contributing to the Delinquency of a Minor (or Providing Alcohol to a Minor): It is against the law for any of the of age adults to supply alcohol to anyone there. They would either receive a ticket or be arrested. (usually when only a minor is drinking though.)
2) Underage Consumption- you get the ticket.
3) There is a little thing called the U.S. Constitution and if the COPS don’t have probable cause (snitching is not enough) to come into that house where you are (loud music- over rowdy crowd-medical emergency) they cannot question you or search the place. What you do in your own home is all up to you.
So be cool and chilll and you should have nothing to worry about.
By the way drinking alcohol on private property is aloud. You don’t even have to be in the house.. you can be on your porch or lawn (just not the sidewalk or tree lawn) and you will be okay… just don’t scream and act a fool or play loud music and piss off your neighbors!
Have fun! (I suspect hang over cures will be the next question?? LOL