The oldest child I have that is still in diapers full time is an 8 year old son.
My 8 year old son is not potty trained and he seems to prefer wearing diapers.
I'm okay with that and his brother who is 5 years old is also not potty trained.
Well I'm sure the 8 year old could use the toilet if he decided too and would likely know how too but it was a struggle trying to get him out of diapers.
So I just kept him in diapers and stopped with the potty training.
If he decides he wants to use the toilet and stop wearing diapers he can.
But until then he can wear diapers instead.