my wife and i went on our first cruise in Sept. We are a young couple and we found the cruise to be great. Not to many kids, lots of couples, some singles, but no "frat party" at all. I am going to assume that it was the time of the year that we went. Kids for the most part are back in school, colleges have started etc, so most of the people on board were adult couples.
So maybe the "frat" image is only certain times of the year and possibly other cruise lines work during the other times. We would cruise Carnival again, but we will also look at other lines to see how they are.
All in all, we were cruising and we were on vacation and if you can just put your mind in the vacation mode and just not worry about everything under the sun. Just have a good time!