Now in my early 40s, I started primary school in September at the age of 4, shortly before my 5th birthday. Those with birthdays after Christmas started four months later in January. I can only remember vague details; a large train set, playing with building blocks, lining up outside the classroom and sitting quietly whilst listening to our teacher.
I don't remember anybody being traumatised by this, or 'not being ready for formal teaching'. I do remember that we all learnt to read, write and do simple sums. I can remember landmarks such as learning my five times table a couple of years later and doing hundreds of addition and subtraction sums from a large textbook. I think that we learnt most processes like this; hardwiring methods into our brains by repetition, which is probably why I can pick up a pen and do a long division sum without thinking today, despite not having done so for decades.
My parents probably gave me a head start as did many others. I don't recall ever seeing any of the examples frequently quoted by experts of children who cannot line up, sit still, or learn to hold a pencil without throwing it on the floor. We all learnt to do these things simply because the teacher told us to, and any mucking about was discouraged by a slap on the legs.
There is much debate at the moment about the Cambridge Primary Review, whose 600 page report calls for a raising of the age that children start school and a delaying of what they call 'formal learning' (and I would just call 'teaching') along with the abolition of SATS (to avoid 'teaching just to pass the test') and a broadening of the curriculum. (There is a constant battle between those who want to concentrate on the '3 Rs' and those who want to teach more broadly) The report claims that in other countries, children start later but apparently overtake us by age 11. Maybe other countries still slap them on the legs if they don't sit still.
You can read more learned opinions about it here or here.