The itinerary won't matter, as far as the age range is concerned. Carnival appeals to a wide range, from families with children to young adults to older couples, as does Royal Caribbean. You might find a slightly higher percentage of young adults on the shorter (3- and 4-night) cruises than on the weeklong ones. That said,you will probably find a good number of young adults on Carnival and Royal Caribbean, as Marian said. I second her suggestion to read cruise reviews (http://www.cruisereviews.com
is a sister site to this one, BTW).