There's no age limit to wearing diapers or using diapers if you need them.
However a child should be potty trained before 5 years old because the child cannot attend or go to Kindergarten when wearing diapers.
A child may be accepted into Preschool when wearing diapers but they must be in the midst of potty training and should be wearing at least pull ups.
Some children may have Autism or other health issues that make it difficult for them to use the potty or learn to use the potty and in those cases the school will accept the child while wearing diapers but they will be put into a special ed class.
In the special ed class the teachers are more trained to deal with issues such as a child wearing diapers and the nurse will change their diapers.
But if a child has no disabilities or health issues that require the child to wear diapers then by the time they are getting near 5 years old then that is too old to wear diapers.
The child should be potty trained by age 18 months to 3 years old but some take a bit longer.
I would not worry about the child wearing diapers still until they are getting near 5 years old and then in that case you need to force the potty training issue.
If the child needs to wear diapers at night past age 5 then they can and should do so but during the day they should be out of diapers and using the toilet.