My husband and I are the same ages, and we cruise with Carnival mostly. They are the closest you are going to get to a "frat" environment. The Fantasy class ships are going to be more of your party crowd as well as 4-5 day cruises. The Fantasy class ships include the Fantasy, Ecstasy, Sensation, Fascination, Imagination, Inspiration, Elation, and Paradise. They are great ships, the Inspiration and the Imagination have also undergone recent upgrades which include an adults only area as well as a resort/waterpark style pool area on the back of the ship. If you have any questions let me know and good luck with your search, I hope this helps!!
I have been on several. I have went with my best friend and myself and with my kids but I strongly recomend Carnival... They do not say that it is the FUN SHIP for nothing. It is a BLAST there is so much to do for any and all types of people at all times... I love it and am going to be cruizing this year and I also will be on Carnival.... Have fun and enjoy.