I think one of the problems is that I don't think enough deferred children have been through the school system yet to be confident it does give them an advantage long term.
Is there any research tracking it in numbers, rather than just anecdotal evidence?
Choice can be a great thing, but sometimes it's easier not to have that option!
For my dc, dd1 started about right, dd2 would have been fine starting the year before, maybe even better (although she then took a gap year before uni so has effectively dropped a year there) and ds was about ready for school summer term in year 1. Girls are both winter babies, ds a summer.
If I'd had the choice, would I have sent dd2 early? I don't think so. But I'd have agonised over it, and I think I'd have wondered several times over her school life, whichever I chose, and felt guilty.
I would have kept ds back a year without a doubt... But now aged 17yo I'm fairly confident it would have been the wrong decision over the long term. He might have done better academically, but I am certain socially and mentally it would have not done him any favours. I'm glad I didn't have the choice.
I think it is something that needs more research. Individual cases, are just that. And some things that are quoted, if you look into it, isn't quite as it sounds.
For example the number of winter term birthdays at Oxbridge looks very high - but if you look into it, a significant proportion should actually be at least one year below. When you take that into consideration, the year looks pretty even. So you could draw the conclusion that for a bright winter term baby it's better for them to go early.
Or maybe that children born in September to November brains develop better. I remember the shoe shop telling me that children's feet grow more in the spring, so maybe there are times of year, like in the animal kingdom, where their brains develop better.
Maybe there's a reason why Valentine's day is in February! It's the ideal time to get pregnant for your baby's development!
Also when I went to school they had 2 intakes. Winter babies started in September. Everyone else started in January. I think there's definitely advantages in that extra term, but I think people nowadays would say that is unfair on the January starters. The winter babies get a whole term extra in school, and have more individual attention.
Shortly after I left, they changed it to all starting at the same time. I knew the head who stayed for about 30 years at that school, from before my primary school time and afterwards. He commented that in his experience although in theory the winter term had an advantage at the beginning they often didn't keep it.
He said typically the January starters arrived and were determined to catch up with the September ones. They followed what the September ones did, so needed less time telling expectations and wanted to do what they did, so often had overtaken the September ones, who sat back on their laurels, fairly quickly. But he did feel that the extra term at home to develop and grow was important, and then they arrived ready to learn.
For me with ds, I wondered if one of the reasons that summer boys do worse is because the expectation of them is lower. I was still being told in year 4 that he'd grow out of certain issues "because he's a summer boy".
He was diagnosed with ASD and ADHD in year 8. For comparison, the boy in his form who was similar, but 5 months older than him, was diagnosed in year 1. That's 7 years extra help. That's going to make a difference.
So what I'd say is if you send him, don't be fobbed off with "he's a summer boy; he'll grow out of it." But if you defer him, don't let him relax and think he doesn't need to learn because he's currently ahead.