30 years ago DMiL, a reception class teacher, when children didn't start school until they were 5 told me she had, over her teaching years, had three or four children start school in nappies. Each was usually the youngest in a big family where the mother didn't want to lose her 'baby' and kept them babied until they went to school.
She used also to speak of children who had been 'talked over, talked at and talked through but never talked to'. She lived and taught in a small country town in North Bucks and towards the end of her teaching career when the council bought up a number of the little Victorian cottages that surrounded her school and used them to house problem families, she would discuss being expected to get children reading in the reception who started school never having seen or held a book or pencil, unable to see the difference between different shapes or with the vocabulary to understand concept like under, over and between between.
DGS, has only just been potty trained at rising 4, he starts school in September. This was on medical advice as he had bowel problems and the specialist felt it was better to deal with these first and then deal with potty training. As it was he more or less trained himself over about a week, while still dealing with the last few months of his bowel problems.