Yes I have seen an older kid past age 4 playing outside with just a diaper on.
My neighbor across the street has a 7 year old autistic boy who is not severely autistic but slightly autistic and not potty trained.
When it's hot out or warm out the 7 year old boy is outside in his front yard or even back yard wearing just a diaper or sometimes a diaper, shirt and shoes.
His parents try to make him wear clothes over the diaper but he won't when it's warm or hot outside and I told them that it didn't bother me.
He's perfectly fine to be outside wearing just his diaper or diaper and shirt.
His parents take him to the park and other public places wearing just his diaper and shoes and shirt sometimes as well.
As a kid I rode the bus with a boy who was disabled and he would rip his clothes off when it's hot or warm out and so sometimes he would be on the bus with just his diaper, shirt and shoes or sometimes just the diaper and shoes.
He went to school that way as well sometimes and even at home he would be out in his yard wearing just his diaper and shoes or diaper, shirt and shoes.
He was 10 years old that time.