This is interesting, I have two summer born boys, July and late Aug. One of the reasons we decided to home educate for a few years was because ds1 (late Aug) was not ready for school at just four.
We kept the boys out of the school system.until they were aged 9 and 6 and they went into yrs 5 and 2. Ds1 has just started a levels, he got all A*s and A's in his gcses, despite being youngest in his year, he would have been bored senseless if he was held back a year and socially etc he would have struggled I think, Always been like an old man in a child's body.
ds2 has just started yr 9, academically in top sets and doing well but he has some health issues and asc, he is very immature for his age, but in other ways his asc means he communicates like an old person iykwim. So not sure if he would be better off in a year below or not!
Ds4 is March born and fine.
Whereas ds3 is Dec born and still struggles at times but he certainly doesn't need to be in a lower year group and socially that wouldn't work I don't think.
Dd is also Dec born and has just started school, she is ready.
As an aside I am ten wks preg, all being well baby due April, my babies are always late so I assume this one will turn up end of April. If I can defer a school place and I think it would benefit then of course I would delay.