I'm guessing a reason that Halloween cruises might have more preschool and toddler children and fewer teens is due to school schedules. It's easier for parents of littles to take a vacation any time, while teens have school concerns as well as more sports and activities that they cannot miss, especially in the fall when HOTHS cruises occur.
FWIW, we took our 9-year-old on
DCL the last week of September last year (she has a funny school schedule and was off that week) and she made a nice friend, but the sailing definitely wasn't crawling with school-age kids or teenagers. (That said, this was less than 2 months after the restart, and I think there were fewer than 1,000 passengers on board.)