The drinking age on all U.S.-based cruise lines is 21 -- even when sailing international waters. (Under special circumstances, some cruise lines allow 18- to 20-year-olds to purchase and consume alcohol with a parent's consent.) Regardless of the policy, you should never buy a drink for someone under 21.

On a cruise ship in international waters, the minimum drinking age depends entirely on the cruise line's specific policy and the ship's point of origin, rather than a universal maritime law. For most U.S.-based lines like Carnival, Royal Caribbean, and Norwegian, the drinking age remains 21 even in international waters. However, many lines—such as Costa, MSC, and P&O—allow passengers aged 18 and older to consume alcohol on cruises departing from non-U.S. ports (e.g., Europe, South America, or Australia). Some lines like Azamara or Celebrity may allow 18-to-20-year-olds to drink if a parent signs a waiver while in international waters. Always check your specific "Guest Conduct Policy" before sailing, as violating these rules can result in the confiscation of alcohol or, in extreme cases, being removed from the ship at the next port.