hey! im ainsley and i have been on 9 cruises. just as long as you are 21 years old at the time of sailing they dont care how old you are at the time you book it. like mike said if anyone is under the age of 21 make sure you have a 25 year old in that cabin with them or otherwise they will be unable to go. also mike is totally right about the entire club/pool/bar/drinking thing..but each cruise is what you make of it. if you want a bunch of people to hang out with that is your age it should be pretty easy especially at night in the club. dont act like you're too good to get out there and dance and act like a total idiot because thats the entire fun of it.
this past spring break my friends and i did a spring break cruise on norwegian and there were a ton of frat/sorority passengers. anyways i hope that helps!