I have cruised on NCL many times and I found that the age demographics are about the same as you would find on Carnival or Royal Caribbean. The average age being somewhere around 50. (Remember, an average means there are as many under 50 as over.)
Usually, the longer the cruises will attract more older cruisers.
There are pool games, rock walls (on newer ships) and many other activities on NCL. I found it about equal to Royal Caribbean.