We brought daughter's boyfriend, who turned 21 the 2nd day onboard, and didn't need any documentation other than his passport. However, Customs at the airport did question why we were 5 people travelling together "as a family" when we all had different last names. ( in Quebec, women use their maiden names on all legal documents; then my husband's name, my kids have my ex-husband's name, and the boyfirend of course, has another name.) I would definately bring a letter from the younger teen's parents though.
Have a great time!