At our school children are fairly quickly sorted onto different 'tables' for differentiated work. Tables usually have names such as colours, animals, football teams or whatever. Usually different tables for literacy and numeracy work.
From Yr3 children are set for literacy and numeracy - two classes in each year group so one teacher takes higher set for literacy and lower set for numeracy, other teacher has lower set for literacy and higher set for numeracy.
I've seen the statistics, but based on my own observations as a student (long time ago) and a parent of a summer-born boy, I really don't think the age thing is as significant as many people think. I've helped out in classes with all 3 of my children over the last 10 years, and honestly the 'top' tables seem fairly mixed as to autumn, spring and summer birthdays.
My youngest (July birthday) is on the top tables for both literacy and numeracy, and those on the same tables as him include 2 June birthdays, 2 April, and 2 in the autumn term. My other children are Feb and March birthdays, and again there seemed no obvious correlation in their classes, both in terms of academic ability and emotional maturity.
Funnily enough, the 3 children I can think of who really really struggled with the whole settling in process were September birthdays, but I think that was just the way they were. Similarly, there is one girl we know who is an Aug 31 birthday and she has always seemed much much younger than everyone else. Again though, I think that is just her and her personality, not particularly the age thing. She is just a very 'babyish' girl in almost every respect, whereas other children born only a week or 2 before her seem so much older.