Generally, the answer is no; nearly all major cruise lines (including Carnival, Royal Caribbean, Norwegian, and Disney) require at least one person in the stateroom to be 21 years of age or older. A 17-year-old cannot book a cruise or sail as a solo passenger. However, there are a few nuances. If the 17-year-old is traveling with a group (such as a school trip or a large family) and a parent or legal guardian is on the same ship, they may be allowed to stay in their own cabin, provided that cabin is adjacent to or directly across the hall from the adult's room. If a 17-year-old is traveling with an adult who is not their legal parent or guardian (like a grandparent or a friend's parent), they must carry a notarized Letter of Consent signed by both parents authorizing the travel and any medical treatment. Some European river cruise lines or smaller niche lines may have slightly different age requirements, but for the standard Caribbean or Mediterranean "big ship" experience, a minor under 18 must always be accompanied by an adult. Always check the specific "Age Policy" of your chosen cruise line before booking, as these rules are strictly enforced at the pier.