I come from a country where you can choose when your child starts school. There is guideline age, and most children start within that age, but some parents decide to put their child into school much earlier or later. I.e. in my class the age gap between the oldest and the youngest child was 21 months!
Obviously I've not done any research but just based on my own experience and observation, age has very little to do with academic abilities. Some of the youngest children did very well in academics and sports, and some of the oldest ones were the lazy ones. You either have talent/ability, or you don't. Or, most likely, you are somewhere in the middle. It also depends on parents - ours were very militant about homework, etc, so me and my sister did very well. Others were much more blase about it, and then were shocked when their 16 year olds were at threat of dropping out of school due to underperformance. Age has nothing to do with it.
If it's so important that the children are roughly same age, and there is a cut off point, why don't they just move the cut off point a few months earlier? So instead of 1st of Sept it would be 1st of May for example. This way the youngest child in Reception would be 4.4, not just turned 4, and oldest 5.3, which isn't that old. I think that would be a much better solution to children starting school when they are not ready.