Bumping this thread because it's personally relevant. Fourth grade is definitely not too early if the child shows an aptitude and interest in it. Keep it light and fun, and see where interest flows! We cover off advanced math during a "math play" hangout on weeknight evenings for 20-30 mins, often with some sort of tasty baked treat.

#nerdheaven

Our household loves AOPS for this.

What is to give light must endure burning.