I've been trying to write a book for years about a post-apocalyptic society entirely run by children, in which all of the adults died off and the children have to figure out how to survive and, eventually, rebuild. However, I cannot figure out a plausible way for all of the adults to have been killed off. Maybe by some sort of sickness? I'm not sure how I'd just kill off adults, though. Any ideas on what might plausibly kill off the entire adult population?
These people can (and must, for the story to work) grow to be older than 18. The thing can move on or still exist, it doesn't really matter as long as it doesn't kill off the remaining children in mass quantities.