Ok so im 21 in college lets say i go to a house party and therea underage drinkers but i didnt buy alcohol?

Why would you get in trouble? You are 21, which is legal age to drink. You are drinking on private property, so you wouldn't get a public intox, even if you were blacked out. You didn't provide any alcohol to minors. The only way you could get in trouble is if it was your house, because you would be allowing underage consumption on your property.