Potty training for a child can begin as early as 18 months of age if the child is ready to begin potty training and get out of diapers.
But some kids aren't ready to potty train until age 3 or almost 4 years of age.
You should at least begin potty training a child by age 3 and the child should be fully potty trained and not need diapers during the day at or after 4 years of age.
If the child is not potty trained by 4 years of age then the parents really need to force the issue and get the kid potty trained because the child won't be able to go to Kindergarten if they're still in diapers unless they have some other problem.
During the night some kids have trouble bed wetting and may need to wear diapers at night to keep them and the bed clean and dry so the child may need diapers longer at night but during the day the child should be out of diapers by age 3 but no later than age 4.