I really liked Royal Caribbean. But I went and looked up Carnival's age restrictions because I went with a bunch of friends last year and we were all under 25. It said

Carnival shall refuse boarding to any Guest under the age of twenty-one unless: (1) the Guest is traveling in the same stateroom with an individual twenty-five years or older; (2) traveling in the same stateroom with their spouse; or (3) traveling with a parent or guardian in an accompanying stateroom. Proof of age and/or proof of marriage are required. Carnival shall not be liable to make any refunds or for any damages with respect to any Guest’s failure to provide proper proof of age or marriage or otherwise comply with this provision.