Carnival is the one known as the "party" boat.
As others have said, what do you mean by "young people"? Here's some of what I gleaned while booking our recent cruise...If by "young", you mean early 20's and wanting to party, Carnival is for you. If by "young", you mean 0 - 12 year old kids, many cruise lines are good, but Disney Cruises seems to have the best reputation with that age group--and not just because of the Disney characters. If by "young", you mean teenagers who will find stuff that is geared to younger kids to be dorky, you probably want to avoid Disney (for obvious reasons) as well as Carnival (because of the influence of the drunken 20-somethings on impressionable teens), and go with someone like Royal Carribbean or Princess (and also concentrate more of your decision on the shore excursion destinations). If by "young", you mean anyone who doesn't need a walker to get around, most will be good--but stay away from the long duration cruises. Anything more than a week, on any cruise line, tends to get more geriatric. Also, three/four day cruises on weekends tend to be more 20-something party cruises, three/four day cruises on weekdays tend to be more family-with-kids cruises. Five to seven day cruises tend to be a widely (and nicely) mixed bag.