Many decades of the lived experiences of children in other countries disagrees with that research. Children in eastern European countries learn advanced concepts from as early as second grade. In asian countries they are introducing Algebra in 6th grade or earlier. This is the problem California finds its self in. They have wonderful "research" showing that kids pick up the concepts better if you wait till they are older, but they ignore the lived reality of the rest of the world, that with the correct teaching you could actually have them doing this work at a much younger age.

Even here in America, tutoring schools such as Russian Math or Kumon are successfully teaching advanced topics to children at much younger ages than the public schools. I know the temptation is to just say its its high aptitude kids that are being sent to these after school programs, but it's not. It is just rich kids whose parents want to give their children a leg up. Almost all children could learn the topics, but we won't put in the effort