I have to say I have been lucky enought to work in some good schools that have made allowances for younger children in a good and helpful way.
My DS is Sept born, making him one of the eldest, and i worry about the expectations of him, having worked in schools and DH is a teacher, as he has verbal dyspraxia and COULD end up struggling to read etc. He has just started school nursery and I opted for the 2 and a half days, mainly so it wasn't a complete shock to his system next Sept, having not attended nursery at all and only being with me. He aslo has some "traits" and quirkyness and would benefit from realising he stays for lunch and understands the whole day. I am lucky in that he is loving it albeit he is very very tired. But like I say I worry they will think he is a bit dim
being a sept boy 
But y nephew is Oct born and my Niece Nov born, he is very clever, racing through (almost 16 now) he is 13 and struggling beyond belief in many ways, if no one knew, one would say he is was a summer born.
So it does vary by child, BUT, you can often spot the summer borns and they DO (mostly) need a good school with a caring relaxed atmosphere that can help support this.