We are going on Ren to Tahiti in Aug. Our first cruise with Ren. Don't know about the younger, more lively crowd, but dress is more casual - no formal nights. "Country club casual" is what they say. Ren doesn't have the exciting night life that most cruise ships have. Our itinerary is port-intensive and everything I've read said no one is up after 11 pm -- too tired. I'm not sure about the age, I've read everything from mostly 30's to 60's. Guess you have to decide what you want. Check out all the different message boards to get a good feel for the ages on board.
As for me, I am greatly looking forward to being in one of the most beautiful places on the face of the earth!!!