@Monkeybusiness09 I have seen this in the USA where many mothers get no mat leave at all.
The ISC boarding school survey gives the real answer to "who does this from the age of 3" in the OP, which is that in the UK, no one does this - in 2023, out of 554,243 boarders in the UK there were no children registered at 3 or at 4, 1 child registered to board at 5, 2 at the age of 6 and around 100 at the age of 7.
The jump is in Year 7, with 1,734 girls and 1,518 boys boarding.