Unless the child has disabilities that make it necessary for them to wear diapers or they have bladder issues then the child should be potty trained and out of diapers fully by age 4 at the latest.
When they get to 3 years old then you really should begin potty training them because it can take 6 months to a year for the child to fully get potty trained and not have accidents in their pants.
Some children may not be interested in the toilet until around 3 years and 6 months which is okay as well but by age 4 they really should be using the toilet.
If the child is getting near 5 years old and is still not potty training then you really have to force the issue and remove the diapers and put underwear on them and deal with the messy and wet underwear until they use the toilet.
Some children will start potty training as early as 18 months and be out of diapers fully by 2 years old but when they get past 4 they are getting too old to be wearing diapers and not using the toilet unless the child has other issues such as bladder issues or disabilities requiring the child to wear diapers.