I haven’t read all the posts but as a mother of an August boy I’ll tell you my experience.
When DS was starting school it wasn’t an option to defer in the way it is now. If I could have deferred him, I would have.
Being the youngest in his year has been really hard for him.
He’s bright, so has held his own academically. But in other ways it’s been a curse.
In reception he was teased for having a Thomas the Tank Engine swimming bag, when the other kids had moved on to Spider-Man.
He's not loud and confident by nature, and being the baby of every class he was in just made him even less confident.
As he got older, his peers did everything before him - got interested in girls, went through puberty, learned to drive, drank alcohol etc.
He’s just gone off on an all-inclusive lads holiday and he’ll have to have a kids wrist band at the hotel because he’s not 18 yet. His friends went clubbing after A levels but he couldn’t go (fake IDs are always recognised and confiscated these days).
Without doubt his life so far would have been easier if he’d been the oldest in the year below. He’d have been more mature, so would have achieved more academically I think, and would certainly have been more confident socially.
This thread is no doubt full of posters saying their summer-born kids (often girls) thrived, but it wasn’t my experience. You know your child, and I can categorically say that my DS 100% should have deferred. I’ll always be angry that it wasn’t an option for him.