A 10k race for a 13 year old pushes the upper limit of what I feel is distance appropriate for that age. A jump up to 26.2 miles and the high mileage training associated with it will most definitely over stress a child's developing body. I cite the young elite level gymnasts that are suffering physical problems today as young adults due to the inordinate amount of stress placed on their bodies at the same age, regardless of physical development. Many adults do not raise their mileage levels to the minimum appropriate level (40+ miles per week) until after several years of running. Separately, almost all of the marathon race directors that I am familiar with limit their fields to entries aged 18 or older.
This represents the minimum acceptable age of what is determined to be safe and reasonable. You can also reference the Usatf sites for age related Junior Olympic race distances. The Usatf is the governing body for all Junior Olympic distance running competitions in this country. For example, the Usatf National Cross Country Championships are held at a distance of 3k (1.86 miles) through age 12 and move up to 4k for ages 13-14. These distances have proved to be both challenging and rewarding to the children and young adults through my first hand observation.
Tip submitted by Bob Pelletier.