All kids should be potty trained by age 4 at the latest and delaying potty training even after age 4 can be considered child neglect and the kids could be taken away from their parents and the parents could go to jail for child neglect.
All children entering preschool or kindergarten need to be out of diapers and potty trained.
Potty training should begin as early as 18 months of age if the child is ready and shows interest but some children are not ready for potty training until as late as age 3 or 4.
Once the child reaches age 4 and still shows no signs of interest in the potty and being out of diapers then the parents need to force the issue and take away the diapers and deal with the messes until the child is using the toilet.
A child needs to use the toilet otherwise the child will face problems in society such as teasing from other kids and being made fun of.