The funny thing is, my 10 year son can do that problem now. The pythagorian theorem fascinated him from the time he first saw it a year and half ago. He is now discovering the joys of the Law of Invesre Squares (laughing).
And he reads HS level literature.
My daughter, soon to be 13, will have to be throttled back pretty soon for evrything but math. She is already doing High School level literature, science, and social sciences. I expect the math to come around - she is now seeing a tutor once a week. She will have to throttled back because our community college won't let her take classes there until she is 16.
You might try EPGY, Stanford's self-paced math program. We've begun our kids in 1st grade. Our fourth grader is enrolled in 7th grade math.