Whenever there's a question about deferring school start ages, it does get me thinking about the other end of Primary.
In England, most start Secondary at 11, with some deferred children just turned 12. Theoretically the oldest could be 12y5mo, if an April birth is deferred.
Then Scotland... used to be 11.5-12.5ys as a start date, but now could be 11.5- nearer 13.
I gather Republic Ireland is also 13 for starting secondary.
The US had Middle school (11yo roughly?) And High School (14yo?)
I felt for my DDs that the model of one class and teacher for every subject was outgrown at 11, in their last year of Primary. Plus quite a few are adult height by then. I find it hard to imagine them being at Primary for longer. But on the other hand... does keeping them at Primary school extend their childhood a bit more?
I do think England starts school a bit too young, and it would be better for Reception to be part of Nursery since they are both technically Foundation stage. But this is more about whether we are getting it right at the other end, and which environment would be better for pre-teens