In Lazlo Zalezac's "Ed Biggers", Part 6 Chapter 13 we have Betsy Carter aged about 3 and willing a "thousands of motors race" (5000m) at a high school meet.
A few weeks ago a 9-year old called Kade Lovell was taking part in a 5000m for kids (St Cloud, Minnesota) and was accidentally diverted into a 10 000m for adults and won it by about a minute. Given that the runner who finished second was 40 y.o. woman, the standard may not have been that high.
His mother had been furious - she was waiting for him at the finishing line of the 5000m and was doing some serious panicking before someone said they'd seen him running in the other race.