How Young is "Too Young" for Chronic Traumatic Encephalopathy?

Right now, no one knows how many hits…how frequent the hits…or how hard the hits have to be before “several concussions” becomes “irreversible brain damage.” The only thing that is clear is that we need to do more to protect the brains of young athletes. If chronic traumatic encephalopathy was found in the brain of an otherwise healthy 21-year-old man, when did it start? In high school? Junior high?