If a child is traveling solo or as part of a group with no member of the group 21 or older, most cruise lines require a written consent letter from the minor’s parent or legal guardian indicating that the child has permission to travel alone.
This consent normally allows the child to receive medical treatment while onboard and releases the cruise line from liability if an accident or injury occurs.
Some cruise lines have a pre-written consent form for parents or guardians to sign and attach a copy of his/her driver's license or other government-issued identification that includes the necessary information the ship will require once boarded.