Our son is also turning 6 later in the year, November to be exact, but he will be starting grade 1 next year, and is already in grade nought at a prominent private school. To be truthful he seems to be coping, and the discussions we've had with the teacher seem to indicate that he will be alright next year and for many years to come.
Having also read Gladwells Outliers, I am sure that the point he was making was that, yes resources are spent on the more mature children because of their apparent ability to grasp and keep up with their current situation better than "younger" children, however the point he seemed to have been making, and what I got from the book, was that we parents of "younger" children need to be aware of the fact that our children get the short end of the stick, and therefore it's incumbent upon us to ensure that they receive what is just and equitable, given that we also contribute to school fees and the like. Furthermore, it's also part of this recognition of these "rights", that we are then also obligated to assist the school by encouraging growth and age appropriate maturity of our children.
This could be by doing simple things like reading to them every evening, encouraging reading when they are older, playing with educational toys, exposing them to older children to help to learn how to cope with situations that are slightly more "grown up". encouraging them to have pets. Offering them guidance as they progress through life.There are many things that we as parents can do with our children that work for both the child and the adult, especially because no two children are ever the same. However we need to actively participate in their growth and maturity.
So the short answer to you would be to allow your child to go to school, but be made aware that you will be expected to participate in their schooling, which mind you is part of the basics when dealing with any child's education.
I myself am born later in the year, and my experience is that Gladwell is correct about the spread of resources, and if only my parents had had the insight that he gives us in that book, perhaps they could have saved a year's school fees at one of South Africa's more exclusive private schools. Luckily for my child, I'm now the wiser for that experience, and Gladwell has made us all realize this fact. I'm able to say, "thats what went wrong', and am now in the position to remedy it.
Hope that helps