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.
I wonder if we should ask the best runners in the world.
https://ourkenya.com/Sports/Why-Kenya-Runs
"Running culture begins in childhood. Kenyan children run to school, sometimes covering 10-15 kilometers daily by age eight. This is not training in the modern sense but transportation and play. These miles build aerobic capacity in a critical window of development, creating a deep physiological base that European or American distance runners, who typically begin systematic training in their teens, never develop. Barefoot running, which was standard in rural Kenya until the 1990s, develops foot strength and proprioception differently than shoe-dependent running. By the time a Kenyan runner reaches age 16 and begins formal athletics training, they have accumulated 100,000+ unplanned running miles."