Hi Cal,
There is not much that your DJ friend can do to enforce his demand for $120.
Since this is a civil dispute - ie a civil wrong (not a criminal wrong) between 2 persons it will not involve the police , he would have to sue you through the courts to get the money and the fee for lodging a claim would likely exceed that amount. You can be sued at any age, it makes no difference if you are under 18 years. However people usually don't bother suing you unless they know you have the money to pay what they are asking and it won't cost more than they are asking. Civil law covers things like breach of contracts and cases of negligence.
Your DJ friend may have grounds to sue you for negligently damaging his property which would entitle him to a right of compensation for the damage (from you), however he would have to prove that you acted negligently, i.e. that a reasonable person in your position would not have done what you did and that you failed to exercise “reasonable care”. However there may also be an argument that he was contributory negligent in failing to exercise reasonable care to protect his equipment in the presence of 'extremely drunk' teenagers.
The moral thing to do would obviously be to pay him, however I don't think you should be worried about legal ramifications.