It is 21 at least. What cruise line.
You may find that if it is an American cruise line they will follow the US rules but if its a European ship they may only follow US rules whilst in US waters.
Adults who are not the parent or legal guardian of a minor traveling with them must present an original notarized letter signed by the child's parent(s), authorizing the adult to take the child on the specific cruise, supervise the child and allow emergency medical treatment to be administered.
Please note: An individual's age on the date of sailing determines his or her status for the entire cruise vacation.