I had read some pretty harsh reviews, and wanted to set your mind at ease if you have already booked a trip on the Millennium. You will enjoy your trip.
This was our fifth cruise, but our first with Celebrity. We have also sailed RC, Carnival, and Norwegian. We sailed with my wife's mother and her grandparents for the first time, but we were free to do our own thing at any time.
The average age on our cruise had to be 60+! My wife and I are in our early to mid 30's. My wife didn't mind the age difference, but I felt like we were a bit out of place. I would have enjoyed sailing with more people closer to our age. We did meet some very nice people at dinner.