I think that both lines are targeting the same audience- the younger adults and families, but I see all ages on Carnival. They have a history of "attracting the younger crowd" but lately that no longer seems to be true. They have a min age of booking at 21, so its no longer the spring break college cruise line. You should be able to me people your age on either cruise line.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007