My ex boyfriend who was incontinent had a "daughter" who was still in diapers and always refused to quit. It wasn't his actual daughter because she was his ex girlfriend's daughter and he raised her and I have seen actual pictures of her and you could tell she was diapered because it looked like she had a big penis. This was unusual because most kids eventually stop wearing them and she never did after years of them trying to potty train her and her refusal to cooperate. According to my ex, his ex girlfriend did drugs when she was pregnant so the doctors were speculating her drug abuse attributed to her daughter's refusal to toilet train. My ex saw her as a DL. This story might sound unbelievable and it does sound like something you would read in a ordinary ABDL story but I saw photos on his laptop he still had of her daughters and I could tell the oldest was diapered. I saw her Facebook page and I couldn't even tell she was diapered so either she finally quit or she has learned to hide them and be discreet about it.
I think it is possible for a parent to be wearing diapers for a medical need and have a kid in them too for medical needs.