An easier route to the all teen powered city would be to state that the city had no paranormals to speak of until an origin producing event (Toxic spill, meteor impact, dimensional rift, whatever) took place at a location where a large number of teens were in attendance. (like a concert or something) and the effects were only manifesting powers to those whose biology hadn’t achieved maturity. Teens got powers, adults got cancer.
If you want to push the whole YOLO, “live fast, die young” thing you could say that once the infected paranormal’s biology achieved maturity the cancer would begin to take hold.