DD was born end of August and I didn't defer. I regret it massively but I think it depends a lot on the personality of the child and maybe also on the size.
DD is tiny and severely underweight. She is small even for her age group but in her current class almost everyone towers above her. She also isn't as strong as the others and I do think that (more than age perhaps) leads to her getting pushed around a bit (accidentally so far I think, not in a mean spirited way) and not get taken seriously.
She has always been very bright and extremely advanced in her language and comprehension but I think that gave us and her nursery and pre school teacher a false sense of confidence. They all said we definitely shouldn't defer as she will get bored.
Academically she is doing very well but socially and in terms of physical skills I can definitely see a difference between her and older kids. She can also be quite timid and shy, which doesn't help
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Also, now I think, why would she have gotten bored. In many countries in the world kids start school at a later age. It just means another year of less pressure (both socially and academically) and more playing and then apparently because you are slightly older you learn reading etc slightly quicker anyway. But what's the hurry?
In PE she is definitely noticeably worse than all her class mates. In academics she is well above average according to her class teacher but personally I don't think she is in the top tier of her year group. Not that I care about that and I don't push her at all but if she'd been in a lower year group she would have really had an opportunity to shine there, which might have helped her self confidence. And in the lower age group she would have been at least average when it comes to sports or physical skills. And size wise she'd also be average rather than substantially smaller than everyone. So, self esteem is something that I do worry about but if she was a more confident and socially skilled child (or maybe just bigger) then the age might not have made so much of a difference.
So for a child like DD I would recommend deferring.
On the other hand my second dc is also summer born but he's a big and stocky boy. Great gross motor skills for his age. And he's also confident and kind of boisterous. He doesn't shy away from making himself heard. I mean I believe he will be able to hold his own even with older kids so I think he will be fine. Not sure about academics but I believe that is something that we as parents can help with. It's not so easy to help with the things that DD struggles with.