I am 24 and I prefer Princess, Celebrity, and RCL to Carnival.
The crowd is a bit older on Princess than Carnival, but I feel it is classier, not that Carnival is Cancun, but it lacks a little bit of elegance that the other lines have.
I prefer to have less people my age and drink a nice martini with an older crowd than deal with immature teens and college students chugging beer, didn't like it in college and I don't like it on vacation now. i say Princess all the way, go to the Caribbean on the new CP, we are going for New Years. Whatever you choose you guys will have fun!