Children in the 80s and 90s were usually potty trained before 2 years old and some as early as 18 months old.
I was potty trained in the 1990 at age 18 months old and was fully potty trained by age 2.
Now some parents wait until as late as age 3 and 4 and that's okay too but if the child is not potty trained at 4 then the parents really need to force the issue and take the diapers away and put underwear on the kid and deal with the messes until the child is going to the toilet.
A child has to be fully potty trained for Kindergarten however some preschools will accept a preschool aged child as long as they are being potty trained and wearing pull ups.
My child went to preschool and was being potty trained and was still having accidents around age 4 so he had to wear pull ups and he continued to wear pull ups in kindergarten just in case because he was wetting his pants in kindergarten sometimes.