Generally middle age to older crowd, but it really depends on the itinerary. One week or shorter cruises younger crowd, longer cruises older crowd, holiday and Alaska cruises lots of families. The idea of the plus & premier fares are attractive to me. I like the no reservation shows, daily activities, some serious, some silly. Do I like everything Princess does? No. But the positives rule by a long shot. Princess provides a nice relaxing experience.
Princess is class if you want class. But, you will not find passengers dressed to kill. It's more relaxed while still being classy.
Suggest you follow some of the live threads. There is a current live from the Emerald Princess. There was also a very detailed live from the 14 day November Ruby Princess circle the Caribbean cruise.