Same here in Florida. You can be on your own private property, and be under 21, and get cited for possession of alcohol by a minor. You can be thrown out and cited at a bar. Some group here in Tallahassee is trying to get the City Commission to pass a law making it an arrestable offense if caught with a drink in your hand and being underage. Right now, you get cited, and have to appear in court or whatever. Some people want the law to change so that the person is taken downtown and put in a cell. The commission and cops are against it, so it wont happen.
Was probably a resturant or coffee bar. Folk bands don't play in hard core bars or night clubs. Most of the people there were probably under 21.