I'm just slightly above your age range, so I understand where you're coming from. My husband and I sail on Princess, but a lot of the people sailing with them are my parents' age, not my age. In fact, I'd say that most cruise lines tend to have a lot of people 50+. The previous poster is right, Carnival is the exception to that rule, so I would recommend them if you're the type that wants a young night-club scene. You can also usually find pretty good deals with them, they're known as an inexpensive cruise line.

Also, you'll find more young people in tropical climates than you will in Alaska or New England. On a recent cruise, we were on the Diamond Princess in Alaska, and there was almost nobody in what's supposed to be the happening nightclub on board. For us, it wasn't an issue because we're not the clubbing type (we're almost 30 and married now, so we've got more of a middle-aged lifestyle), but it would probably have been not very much fun for someone college-aged. Honestly, I don't think we met anyone who was college-aged, other than the crew, during the entire cruise.

Good point too about booze being extra. It's pretty easy to run up a big tab.