I have a few different thoughts on this, but I suppose the main one is, what is school for when push comes to shove. Is it for education, or just childcare.
I grew up outside of the UK, and when I started school there was a list of milestones that should be met before starting. It wasn't a set of requirements, but rather a guideline, the idea being that normally developing children who hadn't met those milestones would be likely to struggle with many aspects of school, including schoolwork, and would benefit from waiting another year to start.
I think that's a reasonable general approach, there can be a lot of variation in maturity at that age and not all children are best placed to start learning to read, for example, at exactly the same age. It makes a lot more sense to have such a child spend another year at home, or a more supported environment, and let time do its work, rather than trying to teach the same material to children who aren't ready - which in my experience can not only lead to real reading difficulties later, those kids often hate school their entire lives.
It seems to me the emphasis now is more on school as a parental right to free childcare, rather than a child centered right to be educated. So that changes the emphasis.
All that being said, I think a major reason kids are being toilet trained later is that so many spend a lot of time in nursery. Which isn't really individually child-centered either.