It always interests me on these threads how different England is to Ireland. Here we can choose to start school anywhere between 4 and 6. It used to be common to send at 4 but has almost universally changed to 5 where I am (and I think country wide)
My daughter, now 15 was born in early May, At that time, the school principal's advice was if they are born before Easter, send at 4, otherwise (and you could afford it) wait till 5.
I had absolutely no idea whether she was ready for school or not at 4. How would you tell? Especially when I'd have had to apply when she was 3. Preschool teachers said she would be ok going at 4 but it would be better to wait.
Staff in the preschool/crèches in Ireland would be absolutely insulted at the idea that 4-5 year olds would be bored in their care. They are professionals and it is their job to educate early years in an age appropriate way.
In hindsight once she got to school, I realised she was pretty mature, got on great throughout primary so she might have been OK going earlier - but was this just her or because she was older?
In sports teams (which have age cut off in Jan) she is with her friends from her year group which has encouraged her to keep playing team sports and going strong at 15.
Secondary transition also fine.
I know lots of English people reading this will be horrified that she will be 19 doing her Leaving Cert exams (A level equivalent) and will be putting on the school uniform for a whole year after her 18th. But nobody in her circle would bat an eyelid at this.
I think the "done thing" on school starting age changed in Ireland as so many people of my age were scarred by going to college at 17. And it has now swung the other way.
The change happened due to parental choice. The compulsory school starting age has been the same (4-6) for donkeys years.
It is a different culture, a different school system so horse for courses. I'm just sharing this to give a different perspective.