I don't think that any child unless they have Sen/medical problems/disabilities should actually be allowed to start school later.
Problem with that is, SEN aren't always obvious at the point when you're doing school applications.
Take DS1. August born. The closing date for primary school applications was before he was 3.5 yrs. Nursery had mentioned that he was a bit behind the other children in his social / emotional / behavioural development, but had also been saying they didn't think he was far enough behind for it to be a major thing, he was the youngest so immature etc etc. So I was concerned about whether he'd be ready to start Reception at barely 4, but in a wondering if he needed more time to mature way. The possibility of SEN hadn't crossed my mind.
And now? Because of the difficulties he was having at school, having started Reception at barely 4, school have put him down a year, raised concerns about SEN, and DS1 has been referred to a developmental paediatrican. From what the paediatrician and school have said, my understanding is that not all possible SEN will be assessed in a 4 yr old - e.g. ADHD won't be assessed until a child is at least 5. Dyslexia can't be assessed until they've started to teach reading and writing.
Assessing for ASD is something that the paediatrician is talking about for DS1, but if he does have ASD, it's not severe enough to have been obvious to me or DH, and in nursery, the signs that are causing concern to school and the paediatrician were assumed to be down to immaturity rather than possible SEN.
We still don't know if DS1 does have a SEN. The assessments the paediatrician is talking about will take months to happen, and she's said that there may not be a diagnosis to be made. It's still possible that he's just on the extremely immature end of the normal spectrum.
But anyway, some of these parents concerned that their children aren't ready for school may actually have children with SEN that haven't yet been picked up on, rather than just being precious parents who are unable to accept that their children are growing up. It's not always as simple as all that.