It is the case that summerborn children do worse. It's why this is the case that is interesting, and it's not all that clear why.
OP keeps saying it's all about children not being only just 4 when they start school. But this can't be true - "redshirting" in the US is a big deal, and there children only start Kindergarten when they are rising 6 - so all children are at least 5 when they start. There are similar benefits in the US to being the older child in the class. I assume at some point this has diminishing returns - i.e. it seems unlikely that starting at 9 while everyone else was 5 would be beneficial, but who knows.
So that brings us back to why being the oldest, no matter what age you start school, has such an advantage. I think it probably is a combination of:
Informed pushy parents
Being the best is a reinforcing prophecy; if you find it easier to pick up new concepts, you have high self confidence that only reinforces itself
Being good at sport - particularly important for boys to form friendships
OP repeatedly refers to poor long term mental health outcomes & says it's more about secondary school/later life. I have to say given she hasn't provided any evidence of this, beyond saying you have to do an FOI, that I find this pretty dubious. Logically being 4 while everyone else is 5 is a bigger difference and disadvantage than being 17 while everyone else is 18. I think it's more likely that summerborns are more likely to be diagnosed with learning disabilities that they don't have, and this might follow them later on.
I have a summerborn who was born 2nd week of August and started reception two weeks ago. We considered holding him back, but decided not to, based on:
- Him being able to read and write because we taught him from 3. He will (hopefully) have immediate self confidence
- All of his nursery friends starting at the same time. Immediate friendship group and social/emotional support; all of his nursery friends are born May onwards and none deferred. He was substantially older than the other children at nursery, and there's no guarantee they would go onto the same school.
- His brother being at the school and able to provide support
- Knowing about the risk he may be "diagnosed" with a learning disability later on; I think this is unlikely given he has good concentration/enjoys rules; but who knows