Reading some, but not all previous comments, it would seem there is no perfect answer, but that it depends on the child, and the LA. I would add my experience as follows:
DD1, in nursery from 6 months old, from 8am until 6pm. She was a July baby, but because of that semi- "formal" setting all her life, was very ready for school at just 4 years old. Our LA however, wouldn't start her at all in Sept, and then only half days from January, so unfortunately we had to go private to get her full time straight away. She was already reading and writing from her Montessori, so would have effectively been going backwards. She always thrived, and we never had a problem with her being youngest in the class. She was both socially, and academically ready.
DD2, did start at nursery but later, at 1, and then only part-time. I then gave up work, and she was at home with me, although still doing some morning nursery sessions. She was an April baby, and although we did start her at 4+, she was never really interested in formal schooling. Her whole primary school experience was a nightmare, she just wasn't engaged. Having struggled through reception her teachers parting remarks were "at least she have left us being able to read". Poor DD2 really didn't find her mojo until secondary, but now is a top performer, and doing well. Should we have started her later at primary? Probably, but for her later would have meant 6 or 7........
So, my experience, very much child dependent, but we sadly don't have that much flexibility in the current system. It's a shame, because for DD2, that would have made her EY education so much more pleasant.