I really am getting a kick out of this thread. I'm 37 years old and will be running my 30th Palos Verdes Marathon next month. Yes, every year since age 8. Last year I was third overall. The time was pure crap.
When I was eight, nine and ten, I ran the times Briana ran for five miles and ten miles. At age 11, I broke three hours for the first time in the marathon. There were other kids running close to my times, too.
My brother ran his first marathon at age 6. He then ran more races, including marathons until about age ten. Then he became more interested in soccer, surfing and baseball.
There were many CHILDREN, i.e. under ten, running marathons in Southern California in the mid 70's to early 80's. As far as I know, I'm the only one who still runs competitively, regularly. And I love it - from all comer's meets to 5K's to cross country. I still get on the track once a week and still put in the serious miles.
I like it just like many are lifelong tennis players or lifelong baseball fanatics.
Where did all the other kids go? Burn out? Major psychological problems? Different interests? Family problems? Drugs/drinking?
The answer is all of the above. I do not think generalities can be assigned or attributed to the effects of running at an early age. It is heavily dependent on the individual. I do not think running is that unique of a sport that has different or especially dangerous consequences. It would be interesting to keep records on the life of chess prodigies, piano prodigies, etc. to see how they fared in life.
Did all the running screw me up? My wife may say so, but jokingly (I hope). I have two kids, went to very good schools (J.D., M.B.A., B.S.), have a good job, mortgage, dogs. Pretty normal, I think. I'm not sponsored by Nike or adidas and never have been or even close to it. I have not qualified for the Olympic Trials.
Will it screw up Briana? Probably not.
Briana may become the next Pozdnyakova, running awesome times for decades (although perhaps exactly why is debatable)or the next Decker. She may also become the next Julia Stamps or Erin Forbes, or even Kathy Ornsby, or none of the above.
My parents did not push me, as other parents did to other kids. Perhaps that is why I still enjoy running. But other kids who were also not pushed stopped running competitively.
I ran all four years in high school, but nothing spectacular. I did not even break 10 for the two mile, or 4:30 for the mile. I competed for 12 seasons in college, improving my times with great coaching and frequent competition. I continued to improve after college, with more really good coaches. I did not burn out, despite tons of meets and racing just for points.
If one wants their children to be the next David Krummenacker, Meb or Geb or Mary Decker, running a ton of miles early on probably is not helpful. But the exposure to competition, discipline and goals may be helpful, although I think it is a mistake to say only running can provide this. That type of exposure is available in other sports and endeavors, but you may not be able to SHARE as a parent, your enthusiasm for the sport.
If you want your children to enjoy track and racing on the roads, as I do and have, getting the kids involved if they want to be involved is fine. They will be able to see your interest, which probably is good. Competition is healthy, the discipline is good, and there's no sport like running, with a watch that never lies, to keep one honest.