You can un potty train someone of all ages.
To un potty train someone of any age you would put them in diapers or they would wear diapers and then the person would use the diapers for both poop and pee.
No toilets and the toilet should be locked and the person being un potty trained should be reminded to use the diaper instead of the toilet.
Eventually the person wearing the diaper would get used to just using the diapers and not hold it back before they pee.
I've been un potty training myself and not have been fully using diapers for almost a year now and would never go back to the toilet.
I just started wearing diapers and using them for both poop or pee.
I would sometimes try to go to the toilet but then remember that I had a diaper on and that I would use it instead.
Now I go pee or poop without even thinking about going to the toilet.
I've pooped my diaper in public and while grocery shopping and I just pee whenever and not think about it.