It's a good question, and I'm not sure. My thought is that by the time you are 10 or so you should have a good enough overall sense of what math is that you can then learn more about whatever interests you whenever you need it. It's analogous to the way kids learn to read...once you're basically literate, you are then free to read whatever you want. I think the whole idea of forcing kids through 12 years of a fixed curriculum is ridiculous. And THAT means teaching kids at a young age to have a sense of what math is and how to learn it, so they can teach themselves.