Princess crowds are quiet and much older, not many children around, if you like that sort of environment, go on Princess.
RCCL has a mix of both, older, families and younger people. In the Caribbean, the younger people and families are the majority. In Europe, it's an older crowd.
Carnival definitely has the fun ships. If you like to hear music, if you like to see people having fun, Carnival is the line for you.
We chose Carnival for this year's cruise, we're in our 50’s, but don't want to be stuck with the ‘old’ people even though we may be the same :)
There is a ship for everyone.
We loved Carnival 2 yrs ago, and were real disappointed last Nov. Their food really went downhill, both selection and quality. They must have hired a bunch of MBAs to do cost cutting.