I have sailed on Christmas cruises 4 times and always found that is a peak time for families to travel. You get a lot of younger age kids traveling with their parents then. You didn't mention if you were traveling with your family or by yourself. When my daughter was in college she met 2 other college age gals from Australia on the cruise and the three of them hung out together the whole week and had a great time!
Oh, NCL cruises are a mixed age group.
I agree with the earlier post. Most Med cruises and cruises for longer periods of time are usually an older crowd. Younger crowd doesn't have as much time off, but Holiday season is always an exception. Go and have a great time!