My take:
1) Scotth has the best responses on this topic, and they mirror my opinion.
2) Age group records at an early age and then none afterwards could be a sign of burnout, but it could also be that it is easier to be the best at age 6 or age 9 because hardly anyone is doing it then. When you have a bigger pool of athletes by the time high school rolls around, you'll get some competition and then start getting your butt kicked - perhaps that helps with the burnout?
3) Burnout could simply be a matter of enthusiasm too. Hardly any of us are still enthusiastic as adults about what we were when we were 9. I wanted to be a baseball player when I grew up or an olympic diver. Don't really have the fire for those things anymore.
4) I disagree with the opinion that running like the young Briana does is the same as the amount of time put into soccer or another organized sport. For one thing, they don't hold practices 7 days a week. For another thing, even if they did do that, the entire practice isn't spent running at a continuous pace - there's time out for strategy, light dribbling drills, etc. - no where near as pounding on your body as running for 90 minutes or more.
5) People should NEVER think a preteen runner shows promise - too many things to steer them off course by the time real development should take place.