Whether a 21-year-old and a 20-year-old can share a cabin on a cruise in 2026 depends entirely on the cruise line's specific age policy. For the majority of U.S.-based lines like Carnival, Royal Caribbean, and Princess, at least one person in the stateroom must be 21 years of age or older. If the 21-year-old is traveling with a 20-year-old, many lines will allow this, but the 20-year-old will not be allowed to purchase or consume alcohol. However, some lines like MSC Cruises and Virgin Voyages have slightly different rules; Virgin Voyages is an "18+" brand, so any two people over 18 can share a cabin without issue. On many European-based sailings, the minimum age to book a cabin is often 18. A major caveat for 2026 travelers is that if the two individuals are not married or domestic partners, some lines may still require a "parent or guardian" to be booked in a nearby cabin if everyone is under 21. If the 21-year-old and 20-year-old are married, they are generally allowed to sail together on any line, provided they can show a marriage certificate. Always check the "Minor Policy" of your specific cruise line before booking, as being denied boarding at the pier is a common and expensive mistake for young travelers.