Yes, a 21-year-old and an 18-year-old can go on a cruise together, but they may face restrictions regarding stateroom occupancy depending on the cruise line and the departure port. For most major North American lines like Royal Caribbean, Carnival, and Princess, at least one person in the cabin must be 21 or older if anyone else in that cabin is under 21. Therefore, a 21-year-old and an 18-year-old can share a room perfectly fine. However, if they were two 18-year-olds alone, they would generally be denied boarding on US-based sailings. In contrast, many European and Australian sailings allow anyone 18 and older to book and occupy a cabin independently. Onboard, the 21-year-old can legally purchase alcohol on US-based ships, whereas the 18-year-old cannot, although the 18-year-old is usually allowed in the casino and adult nightclubs. It is vital to check the specific "Age Policy" of the cruise line before booking, as these rules are strictly enforced at the gangway to comply with international maritime and liquor laws.