With a child of my own interested in running a marathon, I researched marathon training programs for youth in the US (there are some!), injury rates and how these compare to high school running programs.
Anecdote meet singular data point.
Meanwhile, lifetime injuries from overtraining too young and agonizing pain. So… maybe we both choose medical data instead of what happened to our immediate singular data points.
Which… drum roll… says what the original guy said.