If an eighteen year old throws a party, will they get in legal trouble or will the owner of the house get in legal trouble?

To answer this question we would need to know more specific facts. Did the owner of the home supply the alcohol? Were they aware of the party? If they weren't aware of the party, where were they when it was going on? What is their relationship to the 18 y/o? Was anyone injured? Were there narcotics as well?    Generally speaking anyone who supplies a minor with alcohol can be charged with a misdemeanor offense. If the minor just opted to have the party without the homeowner's knowledge though, that's a bit of a different story. The law generally requires a degree of knowledge or intent (mens rea) to be charged and convicted with a crime. Because of that, the specific facts of your situation become critical in doing any type of analysis.    If you're considering allowing your 18 y/o to throw a party with alcohol, that's probably not going to be a good idea. If you've been contacted by police regarding a party that happened in your home, then your best bet is going to be to consult with an attorney who can try and help you resolve the situation.