I have a 17 year old Himalayan cat...?

and for the last 10 months, she has refused(?) to use the litter box. She just does her business wherever she feels like it. I had 5 litter boxes going at one time so that there was no excuse for her not using one. I keep them extremely clean, so it's not that. She is not confused about where her food is and she is still mentally alert, so I can't figure out what the problem is. She's been vet checked twice in the last 10 months and is physically fine. Does anyone have any suggestions? I'm at my wits end here.