If the 6 year old needs diapers then yes they should wear diapers at 6 years of age.
However if the 6 year old can be potty trained and has no health issues then the 6 year old should be potty trained.
But if the 6 year old needs diapers for incontinence, health issues etc or autism or if the 6 year old wets the bed then yes they should wear diapers.
Barring that the 6 year old really should be potty trained as they must be potty trained to go to Kindergarten.