I agree with the above posters about Carnival, for the following reasons:
1. It's the cheapest cruise. Carnival generally undercuts the competition's price by 10% or more.
2. Due to #1, Carnival will attract a young crowd, especially during school breaks.
3. For ANY cruise line, if you look at the pictures in their brochures or at their TV ads, you will see the kinds of people they want to attract. As I look at Carnival ads, I see photo after photo of young people doing all sorts of activities - that is their target market.
4. Reading reviews for the various cruises, I have found that the positive reviews for the more sedate lines come from the older cruisers, and vice-versa.
All of the above being said, if you are cruising to a lively, fun-filled place, ANY cruise line will have a younger crowd aboard, e.g., we are cruising in September (school's in session) to New England and Canada (for scenery) on RCI (not the cheapest), and I will be SHOCKED if the average age aboard is under 50.
Hope this helps.