BackforGood,
well if the Reception curriculum were geared to 4 year olds (and the Y1 curriculum to 5yos) then why are SB boys unlikely to achieve the EYFS goals? (only about half of them do - and that is despite counting April borns as SB too).
I think you'd find that if reception were actually geared towards 4yos, then LOTS of parents would be up in arms that their 5yo is not being taught/challenged/stretched appropriately.
And in fact, there is such a huge difference between the average maturity and developmental stage of a 4yo and 5yo (and between a 5yo and 6yo) that you cannot really do everyone justice.
(Unless you go radical and say that the 5yo's ability/preparedness to learn, doesn't imply that they should be taught how to read/write/add. And hence not teach them. If they (their parents) want to learn, they can do it outside of school. There is no need for a young child to learn to read, even if they could. The need for age appropriate play (that applies to all children) overrides any need for being taught early.
On that basis, you could argue that reading/writing/counting/adding should only be introduced in perhaps Y2 (or even Y3), and that this would be entirely appropriate for all children, even those who could quite easily have learned to read/write/add before.)
One study once concluded that it is not the school starting age that matters, it is the question if the manner and content of teaching is suitable for the age range that matters. We can (and do, if you count pre-schools, or consider school in France) send 3yos to school and it can be entirely appropriate, if their experience of 'school' is suitable to 3yos.
The problem we have in England now is that the reception expectations are not appropriate to 1/3 to 1/2 SB children (who make up 5/12, nearly half, of the cohort). And I would argue that the Y1 expectations (in terms of sitting still, desk-based learning, lack of play, and also academics) are similarly not appropriate to a large part of the cohort.
Sending a just-turned 4yo to reception would not need much thought if reception were such that most just-turned 4yos will be fine. But it isn't.
Sending a just-turned 5yo into Y1 - same thing.