Being some one who was forced potty trained, forced out of diapers to early, being 3 at the time.
Kids who are taken out of diapers to early, no matter what their age is are the ones who end up having potty accidents in school.
there needs to be a disclaimer to the rule the " kids must be potty trained to start school" There are kids who have medical problems and do wear diapers to school. They go to regular classes like every one else.
Some kids even as young as 2 will rip their diaper off because they don't like the feel of a wet or messy diaper, these kids are ready for potty training.
Having a set age for potty training I don't think really works for all. I think the child their selves are the ones who will know when they are ready or want out of diapers. A child must be out of diapers at predetermined age is a taboo.
Their are kids who do not know until they are older how to control their bodily functions. If a child is still in diapers a 4,5 or even 6 it would be because they have not yet learned how to control those functions. Some may never have full control of them. I myself was one of those kids.