We are in our mid 30s and have only sailed with X. We chose it specifically to avoid things like the water slides etc. as, as others have said, that's not for us and we want to relax! I'll never forget the 'older generation' Zenith couple who were on our table at an officers lunch on our first cruise. Firstly they were astonished that we'd 'somehow managed to get an invite' to the lunch (only Celebrity could tell you that!) and couldn't believe that we could possibly enjoy a 13 night transatlantic as we were so young we would just be bored.
The next cruise we were up until 3am one night drinking with two 'older' couples. Let's just all enjoy our cruises!