No 6 years old is not too old to wear diapers.
Actually all 6 year old boys should be wearing diapers as they have not got good bladder control and can wet their pants easily.
A 6 year old boy should be wearing diapers and when in public or at home when it's hot the 6 year old should be wearing just a diaper or just a diaper and shirt and shoes or just the diaper and shirt.
Yes a 6 year old boy can wear just a diaper in public.
A diaper covers up the private area and it's not indecent.
I was at a car show today and saw a 6 year old boy wearing just a diaper and shirt sitting on a camping chair.
He looked very happy to be wearing just his diaper and shirt and shoes.
I wish I could be like him.
If your 6 year old boy needs to wear diapers or wants to wear diapers or you're diaper punishing your kid then they should and can go outside wearing just a diaper and shirt or just a diaper.
No need for shorts over the diaper as the diaper is good enough.
Just like the boy wearing the diaper in the picture below.
Kids look cuter wearing just their diaper.