My female moggy (16) has suddenly started weeing and pooing in DD1s room. DD1 doesn't seem to mind (!) but I certainly do, the place stinks.
We've had a visiting stray recently (who got the blame for the appearing turds) but although we've shut him out with a Sureflap the poos keep popping up. My feeling is she got stressed and maybe started marking and has just carried on now he's gone.
She's got a range of problems from arthritis to kidney failure and cataracts, I'm having the carpet cleaned this weekend and will put a stairgate on to stop her marking the same spot but is it worth a visit to the vet?
We had this too with an elderly male cat, who for no reason stated pooing in our lounge under a chair.
We basically kept him out of the room cleaned the carpet then sprayed the affected area with white vinegar then anchored a piece of heavy clear plastic over the area, just in case he got in the room again.....it did happen of course but was far easier to clean up.
We had this after we moved house as well and I covered the area with that plastic carpet protector (designed for hall carpets) it worked well.
Sadly your cat is very old and I think if you take it to the vets I'm sure they will just say the cause of the behaviour is age, and very pointedly ask what would you like to do.