I am 24 and have been on the Carnival Conquest and Grand Princess. Going on Royal Caribbean this fall.
While I loved the Princess ship and food, I would not really reccomend it for the younger crowd. As another poster said the entertainment and activities are lacking on Princess. It was indeed a much much older crowd than the Carnival Cruise we took.
I think Royal Caribbean will fit us just well.
Crown Princess - Southern Caribbean 10/27/2007
Freedom of the Seas - Eastern Caribbean 2/24/2008
Radiance of the Seas - Hawaii Circle Cruise - Fall 2008
Grand Princess - W. Caribbean - Jan 2006
Carnival Conquest - W. Caribbean - Jan 2005
Radiance of The Seas - September 2006 - 14 nights R/T Los Angeles to Hawaii