What are the signs of dementia in a cat?

We have always had cats and have three at the moment. The eldest one is about 14 and is fit and healthy according to the vet. For the last year or so her behaviour has been unpredictable in that she wants to be stroked and receive attention but when she gets it she attacks you! There hadn't been any changes in her routine and 'Fellaway' didn't help. We are concerned she may attack GC.

Just in the last few months she has started using our bed to pee on! There are other beds in the house which she sleeps on without incident but if she can get to ours (door kept shut now) she is not to be trusted.

I suggested that she goes to vet but DH says that will just cost a fortune in medication which may not help at her age. Already spending a lot at launderette washing the quilt etc.