We have decided to NEVER do another Christmas cruise. There are tons of kids of all ages, many large family reunions. Now, if you are part of one of these groups, you probably will have a great time. But, if you want a nice quiet relaxing cruise, forget it. The last two years we had suites and normally we have outstanding service but not at Christmas. Our cabin steward had three or four people in all but two of his assigned cabins. He had so much extra work and he was exhausted all the time and not in the best mood. The MDR allowed tables to be pushed together to accommodate large groups. Large groups and lots of alcohol doesn't make for the best situation when you are sitting next to them and can't hear anything your husband says. People say and do some real stupid things after drinking a little too much. People save large numbers of lounge chairs around the pool for "their" group. You can get into a fight if you try to take one even after it has been empty for over a hour. Kids are in the adult pool and they take over the hot tubs. And there are many rude people. I could go on and on but we have done it too many times and with the extra costs over the holidays, we just aren't up for it anymore. Last year the two inside cabins across for us had two young kids in each one. They were wild and slamming the doors constantly. We would come around the corner and almost get knocked over by them as they chased each other around the halls or having pillow flights in the hall. They also ordered room service a lot and just toss the used dishes out in the hall - many adults also do that. Also wet towels. I never once saw the parents around. I did talk to concierge once and she walked down with me but they were long gone.
I guess if you really want to go over Christmas, you should go with low expectations. Although we did do that this past year and it didn't help.
So to answer your original question, you will get all age groups.