I haven't read the entire thread...
My opinion is that the later the kids start school, the better.
For me, the problem is in secondary school, not primary school. And the problem is social not educational.
Once the older kids in the class start to get their freedom to go out and have more 'free' social lives, the younger ones in the class are looking to keep up with the older ones. Think of drinking, for example. Not so bad for a 17 year old to have a drink - what about their not yet 16 year old classmates?? Peer pressure is wicked.
My DDs started at 5. In both their classes the children ranged from 5 yrs 6 mths to barely 4 when starting. I am glad that mine are closer to the older range rather than the younger range.
Same with college - IMO, better to start a little bit older, and hopefully have a bit more maturity. Again, from the social perspective rather than the educational one. Looking back, I think I was too young starting college, and would have benefited hugely being a year older. I might not have abused my freedom quite so badly if I had been a little more mature.