It's very common for 2 year olds to wear diapers.
Although some 2 year olds can be potty trained some 2 year olds are not ready to be potty trained.
In reality kids don't really need potty training as they will eventually get it when they are ready.
If a child is ready to potty train they will potty train within a few days to a week at most.
Even if you were to start the process of potty training a child by 2 years old they may not be completely potty trained until they are 3 years old to 3 years and 6 months.
So in reality if you waited until the child was at least 3 years old or a bit older then the child would potty train much easier.
A child should wear diapers as long as they need too and there's no set age that a child has to be potty trained by.
Some kids potty train earlier and some kids potty train later.
It's much easier and better to just keep the child in diapers until at least 3 years old or even 4 years old and then change their diaper and not worry about them peeing their pants and having to clean that up.