It’s been said a few times but I’ll try to explain again.
The disadvantage does not come from being “the youngest” in the cohort, but of being young - it is an absolute not a relative concept.
As I posted up thread, if, by a fluke, there was an entire class of autumn born kids and one spring born child, nobody would suggest that the spring born child should defer because that child would still be ready for school, even if others in the class were older.
If all summer born children deferred (as would mirror the NI system, where the youngest child is born on 30 June), then that would be preferable as those 2 months do make a difference to school readiness.
If some parents of summer borns choose not to defer, then (a) it remains their choice and presumably they would take into account the fact that some do defer and as such there might be a 14 month age gap rather than 12, (b) other parents’ decisions aren’t really relevant to mine and (c) I’m not sure why it would make a big difference if there are some children who are 13 or 14 months older as opposed to the 12 months older that there would be already.
In most cases the deferred children are only a matter of days older than the oldest in the “right” cohort.
There’s so much hand wringing about this in England because it’s still fairly unusual but in Scotland and Ireland this is the norm.