Admittedly I am a washed up and no good runner, but now that I am carrying 20+ more pounds than I did at age 18 while still being quite active as an adult, it seems like running fast around age 18 is so much easier. You might not have much in the way of aerobic development, but speed comes much more easily at these ages where you are still growing taller rather than wider. It's not far-fetched to think that a kid with decent training from a young age might be great as a teenager.
Cheserek is a good example of a kid who ran a lot and was able to be quite fast from a young age. He might go his whole career just trying to shave a second or two off of what he's already done. And he didn't receive any personal coaching until coming here for high school if I remember correctly.
You have a very small sample size of kids who were actually trained from younger than middle school, but in my experience when you examine the results of those kids you find out that they typically crush their peer competitors in middle school and high school. It's not until they get ready for college that LRC naysayers shine spotlights on them and make claims like they've been burnt out or mismanaged by their parents/coaches. Just imagine if a few of those kids don't burn out or go mismanaged.