It really is when a kid starts school. And I'm talking now about children who do not have an underlying medical condition. Teachers should not have to take time out of their lessons, disrupting learning for others, to toilet train kids because parents are to too feckless or negligent to train their own kids. One of the fundamental criteria when sending kids to school, again barring any mental pr physical disability, is that they are properly toilet trained. So it's no good saying it's no one else business, because it damn well is when it involves going to school. Notwithstanding the fact that it is also pretty damn close to negligence to just allow a child to urinate or defecate themselves when they could be trained. If a parent essentially disregards instructing them in perhaps the most fundamental aspect of basic self care then one wonders what else they are refusing to do. I get people will argue "parents will still change them". Yes, but one of the most important aspects of parenting is teaching kids how to actually take care of themselves and be independent, not standing over and doing everything for them especially for elementary physical functions.