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.
No, exercise is good but the extremes athletes put their bodies through is not healthy; is bad for developing bodies, and the changes and consequences are often not even discussed for any kind of consent. In example i recall the story of a woman dealing with chronic pain because she wanted to be a dancer when she was 6 and her parents just signed her up with no discussion.
Also in contrast trans kids tend to be constantly warned of the effects and changes yet get discouraged, gate kept, gaslit, and abused more often than affirmed.