It depends on the ship and it's amount of open deck space. People tend to stay around the first open deck or the promenade deck if the ship has one. Don't forget to think "higher", as in those little islands of deck chairs up at the top.
Families go on Alaska cruises. Again, it depends more on the time of the cruise. If you go in May or September, more schools are in session and you'll see fewer teens. Go in June, July, August, and there will be plenty of younger people.