My friend's DS was born on 31st August, but was 8.5wk early. His actual due date was 31st October. Despite developmental delay and various recommendations, he started school at 4yrs old (and a few days). He is now Yr1 and he is coming along really well.
My own DD is late August born (my friend and I were in hospital together!) and now in Yr1. DD struggled to settle in nursery; she found any change unsettling and literally cried at every drop off.
I will be honest; I was completely worried about her starting school. We moved in the July before her starting school, so we also had to change schools. We found a lovely village school - our catchment school - and the HT was very approachable and amenable to DD's possible needs. She said we could defer until spring/summer term, DD could do all mornings or 3-4 full days per week - she even said we could 'play it by ear' and work together to find the best solution for DD.
As it was she ran in on the first day, with a quick look back and a 'bye mummy' before disappearing into the class, and she has been like that ever since. She is also ahead in terms of educational achievement; she has been streamed into the Yr2/3 class with a couple of her cohort and is currently on 2b/c/b for reading, writing and numeracy - expected level of achievement at the end of Yr2. Physically, she is still much, much smaller than the rest of her class and she can appear clumsy. But we take her to gymnastics and swimming - activities she enjoys, but that also improve her strength and co-ordination.
If you had asked me 2yrs ago, I would have told you that I wish I'd just kept DD inside for a few short days longer. If you ask me now, I'd tell you that she is positively thriving.
Tiggytape is right; you need to focus on the options you have, rather than mull over those you don't. I completely understand how unjustifiably unfair it seems, but there has to be a cut-off somewhere. It just so happens that the cut-off makes our DC the very youngest.