At 16, an age when most teenagers are concerned with driving tests and prom dates, Kane Parsons made a nine-minute short called “The Backrooms (Found Footage),” about a man trapped in an unnerving realm of mazelike hallways. The film and its follow-ups became such a YouTube sensation that, just months after Parsons turned 17, A24 signed him to direct a feature-length adaptation of “Backrooms,” making him the youngest director in the studio’s history. Two years of development followed, meaning