I can't remember what exactly Royal Caribbean's policy is. They're website is down as of right now, so I can't get on the site to check. My recollection was that at least one person in each stateroom had to be 21. This may be the case for most major cruise lines, but I'm not sure.
2007 - Majesty of the Seas (Bahamas)
2009 - Enchantment of the Seas (Western Caribbean)
2010 - Freedom of the Seas (Western Caribbean)
2013 - Golden Princess (Alaska, roundtrip from Seattle)