When I pulled my kids out of traditional school we started introducing concepts of Algebra and Geometry to them immediately (2st and 4th grades). It takes extra work, but they understand everything just fine. The problem isn't that the kids can't understand it. The problem is our education system is broken and no longer has the ability to teach children anything other than what might be on a standardized test. This is literally every subject, not just math. Writing, history, spelling, science, even physical education has suffered the same decline. The whole of the education system is a shell of its former self, and is more concerned with providing child care rather than instruction.