I have seen older kids above age 3 wearing diapers in public.
A few days ago I was at Taco Bell and eating some Tacos and the kid in front of me at the booth stood up and I could see a Pampers Cruisers diaper sticking above his waistband of his blue jeans.
He looked to be around 7 years old and looked very normal.
I'm not sure if he was incontinent or just was not potty trained and wanted to wear diapers instead.
I also saw another kid the same day in Dollar Tree who looked to be around 6 years old and it was a boy and he was wearing a Luvs diaper.
I saw the waistband and part of the back of the Luvs diaper stick above his pants when he bent down to look pick up some toys from the shelf.