The Carnival Programs are pretty standard throughout the fleet. There is Camp Carnival, for ages up to 14. There's Club02 for ages 15-17. Unfortunately, that leaves out the 18 year old. I have read posts that some 18 year olds were able to join some of the teen activities, and other posts that have stated that they were not allowed.
Here's what the 2005-2006 Carnival brochure states:
Camp Carnival children's program: Supervised year-round activities for young cruisers 2 through 14 years of age; Youth Center with computer and play areas; ExerSeas recreational fitness program; A-B-Seas reading program featuring storytelling, skits and puppets; Arts and Science Programs
Club 02 for teens: Exclusive program for 15 through 17 years of age; Teen club with dancing, music, video games, movies and more; Fun parties, activities, sports and shore excursions; Teen Director aboard every ship.
It looks like Carnival has redesigned their programs since we last sailed, but can tell you that my kids have always enjoyed their Carnival cruises and Camp Carnival experiences. There were activities they could pick and choose from throughout the day and evening. (they sailed with us at ages 11 & 13 - Elation, and ages 13&15 - Inspiration, and are sailing this summer, ages 15&17 on the Conquest!)