There's a book out called *Better Late than Early* by Drs. Moore and Moore, that discusses that very issue. Many children are not physiologically ready for school until 10. The optic nerve is among the last to be myelinated and so the signal is often garbled by the time it gets to the brain. They state that many of the cases where kids are put in special programs to help with there dyslexia improvement came because the child matured physically not because the program worked. But since the child improved while in the program, the program is given the credit.
I grew up in the same small community where the Moores lived and taught. They had a certain following, and they were "interesting characters," but their research wasn't so overpowering that more than a very few of their students followed their lead.