I could bore for hours on this. 😂
Both of my DSs deferred; in Y12 and Y14....
So they're 23/21 now. You couldn't defer back then.
DS1 b. end of May, into R year, then into a split Y1/2, or Y2. (him in the former). In this class were 25 DC; 21 boys, 4 girls. Teacher and I agreed what DS1 needed was to repeat R, 'along with 90% of all my boys' said the Y1/2 teacher!
Anyway, we soldiered on. His mate, S, was Sept born (in Y2 class). He got a 3 in his KS1 SATS. Well done, S! Mine got a 2b. Thence onto junior school. Where the SATS monitoring continued. My DS achieved a 3, aged 4 months younger than S had been, when they were at the end of Y2/KS2, when S got a 3.
But no one celebrated my DS.
So, the considerably older child was celebrated and validated. Like they do if they're 20% older and possibly bigger than their classmates, in sport, drama (Joseph and Mary?) and comprehension in R.
Something like 75% of Oxbridge applicants are Autumn born.