And not that I would be against you cruising, but also don't "lie" about you age and book anyway expecting to be able get on the cruise. They will be checking your ID (passport and or birth certificate) when you embark. I'd hate to see you put money on a cruise and lose it because they won't let you on.
Any way to get a parent, relative, or friend (21 or over) go with you? That way you get to have fun as well as the person you talked into going...
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, Magic 1/14, Freedom x 2 6/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013