Our first Celebrity experience was on the Edge (after sailing Disney many times). It was a great cruise and I was worried I wouldn't like the classic ship experience. We had just as a great time on the Millennium as we did on the Edge. The crew and entertainment staff (Manuel was the cruise director) were excellent. We found the variety acts on the Edge great and the scripted shows not so great and it was the same on this cruise. If you have a chance to catch The Boy Band Project on a Celebrity sailing, do it! They're a lot of fun!
As for age demographics, there's a lot of info that Celebrity is just for the seasoned traveler, for the 60+ crowd. We found the age to be lower on this cruise. There were more Gen X'ers than our previous trip on the Edge.
Food was very good, better than Disney Cruise Line, especially the buffet. There's so much variety and it actually tastes really good. The main dining room is very nice and the food is good. The French onion soup is one of my favorites. We opted for Celebrity Select dining, which means you can go any time between a certain time period. We didn't have to wait, but then again, we ate at 5:30. We requested the same table each time at the very back windows great views of the wake.
Cabin with an oversized balcony. It's a good size for two people, but three would be crowded. The room is close to elevators, but there's no noise from the hallway. We'd stay in this room again. Deck 6 is close to many things and we could take the stairs to most things except pool, Oceanview Cafe, and the spa.