I have 4dc with autism and 1 with learning disabilities. The dc with autism were all trained between 3 years 10 months and 4 years. One has physical disabilities as well so he had accidents at school regularly in reception and year 1 because he couldn't physically get to the toilet in time. At home I would carry him to the toilet or we would have a potty in the room he was in. By year 2 he could hold it in long enough to make it to the toilet.
My child with learning disabilities was nearly 5 when he was toilet trained. He was the only child in nappies in his reception class. Not sure about now as my youngest is in year 6 and I don't know most of the younger ones but there was always 1 or 2 children in my dc primary school who were in nappies at any given time but certainly not 1 in 4. The reception teacher would usually give out 3 or 4 carrier bags of wet pants a day to various parents and the year 1 teacher would hand out probably 2 or 3 a week. The children in nappies would without exception have SEN. About half would be in SEN schools by year 3. All of them would have been in special needs schools in the 1990's and in residential care in the 1960's.
These days there are lots of children (including mine) who would have been in special schools when I was a child in the 1990's. They are now in mainstream but without enough of the help they need. Children who have downs syndrome, moderate learning was etc are in mainstream school, along with children who have a 1-1 TA for their own safety to stop them escaping or climbing on the roof.
When I was at school in the 1990's we had 1 boy who had cerebral palsy and was in a wheelchair, 1 who probably had adhd because he won everything at sports day and could run like the wind but lots of the teachers refused to teach him and he was always taken out on a trip whenever Ofsted was coming (back when that was allowed). A few who needed TA's to scribe for them and explain things and a couple who were deaf. That's it. And there were enough TA's to go round.
I think the main problem here is there are not enough staff to deal with the problems that children in mainstream school these days have. One teacher and no TA is not acceptable in any reception class. For children who are in nappies but don't need any other support there should be 2 TA's for the whole school who go into each class and change nappies when needed. Those who need more help should have a TA of their own. We should have more health visitors, continence nurses and sure start centres.
I know people will say where is the money coming from and the government can't afford it but they have cut early years and education funding to the bone and this is the consequences.