Sorry, ignore my C&P, I have now caught up with some excellent debate and discussion and that point seems irrelevant 
Starlight, I totally get what you mean. Socially, I think children of that age play alongside each other and are beginning to interact but it's that transition, it's not like yr1 where they all interact with one another.
I see the point about the child going into a class where everyone has already learned a lot of things, however they will all still be at different levels and I think it would be wrong to send mine in now as he is just too autonomous.
He won't brook any argument about what he wears, eats or does...he still sleeps almost every afternoon, has only shown interest in school within the last 24 hours...he's not raring to go like so many of his friends are.
I want him to be emotionally and physically capable of coping for 6 hours without my help. That's the entire basis for keeping him out till January. He was born in June though so far littler than a lot of the others. I may even wait till April to start him.