Like Acorn, I can think of several musicians who developed dementia in spite of carrying on playing and singing as long as they could. Two of them would bring their partners to our sessions, and they would act as prompters, or point out which line in a written sheet should come next. They were all guitarists/singers.
I am steadily losing my hearing, including sense of pitch, which makes it harder to sing in tune, but I am soldiering on playing my instruments, as I hope this will keep me from developing deafness-induced dementia. My memory still seems pretty good, but I think part of the association between hearing loss and dementia may be because hearing-impaired people often withdraw from social contact, as it’s a struggle to cope with conversational situations. Sometimes it’s easier to just sit at home on your own: it is extremely tiring, going out and struggling to keep up with everyone talking, and in my case, it aggravates the tinnitus too.