What are an artists rights when it comes to being defamed by a promoter at an event?

Based on the details you provided, you were contracted to perform at a certain place and on a certain day and time. Yu were present and ready to perform. The promoter is obligated to meet his side of the deal. If he failed to properly ascertain the terms of the venue, it's his problem. I'm not sure what you mean by making the equipment "unuseable" but if he deliberately did something to prevent you from meeting your side of the agreement, he is liable. As to whether he actually defamed you or this was just an unfortunate incident, check with a local attorney.

The facts as you described, in my opinion, do not meet the definition of a defamation action. You have presented no evidence that anyone slandered the artist or committed libel by writing derogatory, damaging statements about the artist. What this does appear to be is a breach of contract action, if in fact, there was a contract with specific language as to the age limits of attendees. There are several parties involved in your fact pattern: the venue, the artist and the promoters. The venue is only bound by its contractual agreement. You will need to look at the contract signed by the venue to ascertain whether there was any language at all as to the age of attendees. The promoters may have gone off on their own and decided to advertise "18 and up." So, you really need to review the contract(s) to see who, if anyone, would be liable for a breach.

California law and practice are very big on conducting entertainment business legally and properly, so it is odd and rare that this happened in California. In California, the only one that can legally book you to play a show is the venue owner, a show producer, or a licensed talent agent. In any of these 3 cases, they must be financially and legally responsible in every way. If you get involved with a "promoter," you can be sure you are involved with someone operating outside the law. And as you found out, it is a nightmare. California alcohol licenses for bars, clubs and restaurants are very strict. If it is a bar serving alcohol, the entry age is 21 and over. Sometimes a place that has significant food service also runs shows. They may be able to run an all-ages show during food service time. If you want to run a show that allows in teens, you need to book it at a place that does not serve alcohol. I have California music clients who do this very thing they book spaces and run their own events for teens. If you want to play for the teen crowd, you can also arrange to play at shopping malls, parks, plazas, schools, parties, and other such places without alcohol. I do not think this situation involves defamation. It does involve someone who is not licensed as a talent agent offering you employment in entertainment, which is illegal. You can complain online to the California Department of Industrial Relations. Google on that and you can easily find a complaint form online. After your bad experience, I hope you now understand why the State licenses talent agents and requires anyone booking a show to be licensed and insured. Also, it pays to have an entertainment lawyer to consult with on a regular basis, to guide you along, and check out any contracts or deals BEFORE you sign or agree to them.

If you can show damages, you may have a claim this will require a full legal analysis to fully figure out. You should take any contracts or other documents you signed and go see an attorney in person for a consult.