I would add a dissenting voice here. While I recognise that for some children starting formal education (rather than the current play-based Early Years Foundation) at 4 / 4.6 is inappropriate, for others it is entirely appropriate.
My daughter (a February birthday) asked if she could learn to read shortly after her fourth birthday. We asked a family member who is a reception teacher for advice on the best way to go about it and DD was able to read when she started school in September.
The school then overlooked her particular needs and left her desire to read more without much stimulation for six months, after which they started basic phonics. By this time (around about her fifth birthday) she was reading books at home and the mismatch between her particular needs and the education was huge. Only now that she is in KS1 are her individual educational needs being looked at, and her first year in Reception was largely a waste of time.
If we decide that starting to teach some children too early will frighten them and turn them off education, we risk leaving bright children who want to learn in a formal way will to stagnate until they are 6 ro 7 years old. Why can't we educate children at the level which is appropriate for them, rather than assuming one size will fit all?
Cheers,
PV.