Heres a 12 year study on Go Kart injuries now it does include Fun Karts (Open to the public) It's in PDF format But 19,000 injuries are pretty amazing stats. I had a friend once have his eyes knocked out on a Quad Runner. Do they have to be 18 to ride those too? I've seen people hit trees ect on them.

http://www.cpsc.gov/LIBRARY/gokart.pdf

From the article on Karts.

There were 231 go-cart/fun-kart related deaths of all ages,
reported to CPSC from these sources during the years 1985- 1996.
One hundred and fifty-five cases (67%) were to children under the
age of 15 years old.

The thing about this is the total number of karts and drivers ridden over the period. It would be much easier to estimate the number of sprints and number of races vs. THE Entire Kart world and Entire Kart number of riders.