Some older kids wear diapers even when they have no medical problems because the older kid may have wetting accidents or the kid may be having diarrhea and wearing the diaper is good for protection.
It's much better and more sanitary for an older kid to wear a diaper and wet or poop in it than to wet the pants and poop the pants and get it all over the place.
Also some kids may wet the bed at night and need to wear diapers to keep their clothes and bed dry.
Also some kids may wear a diaper when they go to places such as amusement parks etc where the bathroom lines may be long.
In that case the child may not make it to the bathroom to pee and so the diaper allows them to pee without wetting their pants.
Another thing is some kids may actually have a bladder issue where they are actually incontinent either through a birth defect or maybe they got into a car wreck or something else and they have a bladder issue or bowel issue as a result.
So in those cases they need to wear a diaper.
My nephew who is 8 years old needs to wear diapers full time because he has urinary incontinence and cannot hold his pee at all.
He's never been out of diapers since he was born and my 8 year old nephew has been wearing diapers full time since birth.
He's never used a toilet and probably never will.
He can poop in the toilet but since he's wearing a diaper his parents just let him poop in the diaper as well and they change him.
He's a skinny 8 year old so he wears Pampers size 7 diapers and they work great for him.
Eventually he'll need to start wearing youth sized diapers and adult diapers but for now he fits into Pampers size 7 cruisers diapers and they work great for him.