There isn't an official "fastest" time but at my school (I'm 15) we have a record for almost everything you can think of so you may want to check about that.
13 seconds
21:53
it is 19.23 seconds
The fastest officially clocked pitch was by Brian Jarrell at 72 MPH.
Eric Hulst 14.57.93
It really depends more on the physical characteristics and health of the individuals involved and not the age or whether they are a boy or girl.
Lynn Gum ran mile at 3:59 when 17 years old
My 11 year old brother ran a 5:45 mile at a road race in Shorham New York
i Ran a 11.3 in a meet 5 days ago
I doubt that any serious rigorous studies have been done on this, and further doubt there would be much difference in the average speed. That said, I suspect that in a large enough sample, the fastest boy will be faster than the fastest girl.
3 billion miles
For a 12 year old boy, 6.82 seconds is the average time for a 40 yard dash. The fastest time posted is 6.13 seconds.