18 is quite an age. When you say you tried 'punishing' her I winced a bit - but I know it is easy to misunderstand a poster's meaning as there is no body language or anything. I know from what I have read that unless you actually catch an animal in the act of doing something they shouldn't be, they have no idea why they are being remonstrated with. It sounds like your poor old girl isn't being naughty but is a bit confused.

One of my cats keeps messing on the floor at the moment - and she doesn't have the excuse of advanced age as she is only 13 - so I know how frustrating it can be.

I admire you for taking on such an elderly cat. I rescued one of 15 once - she only lived for six months but cost me £500 in vet bills. It was worth it though to know she had lived her last few months out of a cage and in a home environment. I hope your elderly furball lives a long and happy life with you.