Know that chances are most of the people will be older than you, but honestly, we have more fun with retirees than younger adults anyway.
But things have picked up since then, so I'd anticipate a busier ship and more energetic vibe- especially in the Med (Celebrity is originally a Greek line, so that's kind of their bread and butter). it holds over 4000), so there wasn't a ton of energy in things like the nightclub or for musical acts. When we were on it (last November) the ship was pretty empty (like maybe 1500 passengers.
I wouldn't expect a 'nursing home at sea' vibe, but it's not an oceangoing rave either. Depending on when you go, the demographic may be older, may be younger.