My thoughts? You're not going to get any hard numbers out of me but I think a 24 hour event is extreme. I'd probably be somewhere between the 3 mile limit you are placing on your girl and that kid's 65+ miles, closer to the 3 miles than the 65+. The reason you're not going to get any hard numbers out of me is because it depends so much on the physical and mental maturity of the child as well as, of course, previous running history. Most are probably able to handle more than 3 miles by 7 years old with an adequate background in running, though it may be a reasonable limit for some, but I'd be surprised to find someoneone under 15-20 years old who is ready for 65 or more miles.