Passengers under 21 years of age must be accompanied by a parent, guardian or chaperone who is at least 25 years old; one adult chaperone is required for every five people under 21. Parents, guardians and chaperones are responsible for overseeing the onboard conduct of minors.

Whether a 21-year-old and a 19-year-old can cruise together depends entirely on the specific cruise line's "minimum age for booking" policy, which varies significantly by region and brand. For most U.S.-based lines like Carnival, Royal Caribbean, and Princess, the policy generally states that at least one person in the cabin must be 21 or older, so a 21 and 19-year-old pair would be permitted. However, some lines have stricter "minor" rules, requiring anyone under 21 to be traveling with a parent or guardian. On the other hand, many European cruise lines (like MSC or Costa) and "lifestyle" lines like Virgin Voyages allow anyone 18 or older to book their own cabin without an older companion. Regarding alcohol, if the ship departs from a U.S. port, the 19-year-old will strictly be prohibited from drinking. If the ship is in international waters or departing from a non-U.S. port (like in Europe or Australia), the legal drinking age is often 18, allowing both travelers to enjoy the ship's bars. Always verify the specific "Age Policy" on the cruise line's website before booking to avoid being denied boarding at the pier.