I'm not going to even attempt to put % on each of these, but a combination of:
- kids with the right biological age/development to benefit from...
better training, technology (tracks, shoes, supplements, recovery aids), racing opportunities, belief and knowledge that it has been done before and they can do it themselves
Regarding the biological age - kids grow and develop at different rates. We all had a small kid in class that didn't grow for years, or another kid that hit a huge growth spurt and dominated sports due to physiological advantages from being further ahead in their biological development.
Better shoes means nothing if your kid is a year behind their compatriots in biological development (almost everything in life uses chronological age based on DoB, so take someone born August 2nd and compare them to someone born on July 27th in the same class). But being able to understand and identify this biological development is significant for coaching kids in any sport.