I found this
on Royal Caribbean's website. If I interpret it correctly, I believe at least one member in each stateroom must be 21, unless there's a parent or guardian in an adjoining stateroom.
I like Sandy's suggestion, though, of booking through an agent that has experience booking large groups going on a cruise. They would have the knowledge of the age policies on most cruise lines.
2007 - Majesty of the Seas
2009 - Enchantment of the Seas
2010 - Freedom of the Seas