Throwing a party -- is a "No alcohol under 21" sign legal immunity?

NO NO NO NO NO. you are responsible for keeping the alcohol away from your underage guests. don't have a party with people you don't know, and don't serve alcohol at a party unless you know everyone is over 21. Tell your invited uests that if they bring a guest must be over 21 or they can't come. make sure they know that he/she will have to show ID at the door