The average age that a serial killer starts killing in their late 20s to early 30s. Denis Rader, the BTK Killer started when he was 29 years old. Jeffrey Dahmer started when he was 18 where Dorothea Puente didn’t start until she was 53. On occasion though, people start killing much younger, like not even a teen yet younger. Here are 5 of some of the youngest serial killers.