I think there could be a lot of interesting things to look at.
My 3 dc are all fairly equally bright.
Dd1 went to school about right time (winter baby)
Dd2 would have been better the year before (winter baby)
Ds was about ready summer term in year 1 (summer baby)
But, they're now older and I can look back: DD2 might have been fine in the year above, but I think she would be struggling now. She was diagnosed with anxiety when she was about 10yo and I'm very glad she wasn't being pushed ahead.
Ds would have been better in the year below, or even 2 years below right through infants even until about year 3/4. But I'm not sure how ready he would have been at that age if I'd left him. You see he just takes time to adjust to things, and I think if I'd said he won't go for 2 more years he would still have taken a year or so to adjust. He also would be bored silly by the work, and probably inclined to mess around and get it done.
So for some children it could be initially good, but ultimately cause problems down the line.
I had a friend in US where they could choose to send them any time from age about 4 to 7yo. So the parents of the girls wanted them to be seen as academic. So they rushed them in asap saying they were desperate to start. The parents of the boys wanted them to be in the sports' teams. So they wanted them to have the advantage of age so they were as old as possible. Children who didn't have parents that followed these tended to end up being the oddments. So they had class of reception with 4yo girls and 7yo boys-and parents were pushing for younger (girls) and further delayed (boys).
I've had a number of parents who have said to me that their dc is desperate to get to school, really ready to go, where I have known the child enough to think "no they're not". So I don't trust parents to necessarily make the right choice.
I think the ultimately doing better is interesting. There was a discussion of birth date and Oxbridge on here once. Winter babies were heavily represented. And thinking through my friends at Oxford that's right, a large number of people whose birthday I knew were September or October birthday.
But. He's the catch. About half of those birthdays were at least one year-some two years, ahead. So not only were they not the oldest, but they were actually younger than the summer borns.
And when I took those out of the question, the summer/spring/winter term babies had the fewest in the spring. The summer and winter were almost equal.
Which leads me to wonder if there's actually a time of the year when the brain develops better in pregnancy. Or maybe there's a tendency of better educated parents to aim for September/October birthdays?
Someone who worked at a shoe shop commented to me once that children's feet grow quickest in spring (was true of mine too) so maybe there's a reason why if you're born in those months your brain development is better.
Another thing I find interesting, is that people will say "oh we push children into education too early." But when I was little children didn't start all in September, you started the term in which you were 5yo.
When dd1 started it had changed so they all started part time in September and went full time the term they were 5yo. When ds started all went full time in September.
But I'm not sure it's helpful. I remember the teachers at primary saying to dm that often the later starters did better because they saw the older ones there and wanted to catch up so worked harder. I certainly think for ds, easing him in by going part time for 2 terms would have been better. It was arranged when they did that so that the part-timers still did all the academic work, but didn't stay all day.
I wonder if it's sometimes expectations. When dd1 was in infants whenever the class did anything public the teacher would say in the how brilliantly they've done speech "and remember some of these children are only 4yo" (or 5yo in year 1/6yo in year 2). Taking to dd1 and saying how well she'd done she replied "no, I did what was expected. It was who did well because they are only "
And with ds, I often had "oh you know he's only 4yo/5yo, he's a summer born boy, he'll learn when he's ready."
So do people expect less of the summer borns-and make the winter ones feel they should be achieving. And are people less inclined to intervene early, which can be important, on the younger ones because they will grow out of it.
I suspect if ds had been a winter baby behaving as he did in the winter term of year 1, when he was a new starter in year R (so older than the winter babies) then they'd have been talking at looking at getting him assessed. however I was being told "he's a young one, he'll grow out of it."
Luckily for him, he pretty much has, but others haven't, and it can make a difference.
I think there's a lot more too it than simply just birthdate.