The appropriate age for a child to stop wearing diapers in public or anytime is when the child is potty trained and no longer needs diapers.
There's really not set age or appropriate age that it's either acceptable or unacceptable for a child or anyone to wear diapers in public or anywhere else.
However by the time a child is 3 or above then it becomes less appropriate for the child to be out in public wearing just the diaper.
But under pants or shorts then it's still very acceptable for a child to wear a diaper in public when they need one.
Diapers are for adults and babies and toddlers and anyone of any age that needs to wear them.
If you or your kid or teen needs to wear diapers and needs to wear diapers in public then they should do so without any shame or fear.
Many kids, teens and older adults need to wear diapers and many do so in public without anyone noticing or caring about it.