Carnival is known to be one of the lower priced cruise lines available, so it does tend to attract a younger crowd than some other cruise lines. A lot also depends on when and where the cruise is.
In General...
You will typically find the younger average cruising ages on: Caribbean cruises, shorter cruises, cruises during school breaks (including summer)
You will typically find the older average cruising ages on: Alaskan cruises, European River cruises, American River cruises, World cruises, longer cruises
Take a 3 day Caribbean cruise over spring break: average age may be 28 years old.
Take a 8 day Caribbean cruise a couple weeks later on the same ship: average age may be 48 years old.
Those are just some generalizations that aren't always true, but for the most part they should at least give you an idea of what can be expected.
Whatever cruise you take there will be others your age to interact with.