My stepfather's old cat Gipsy died recently. She'd been spending most of the time asleep lately, & we think she was a bit demented. She sometimes seemed confused. Sadly, we suspect that killed her: she was found floating in the canal basin. Perhaps she forgot that she could no longer jump across gaps onto boats.
Her oldest known living descendant, Tremi, has since died. She'd become very unwell, losing weight fast. The vet diagnosed cancer. Treatment was thought cruel, since she wouldn't understand it, so she had a last trip to the vet.
They were 19 & 17.