My cat is 12 years old and I have had him since he was a kitten. He is one of the coolest cats around. If you call him, he'll come. If you talk to him, he will talk back. His younger "sister" beats on him, but he takes it well.
A couple of weeks ago we found a lump on his side and we had it removed. Turned out to be cancerous.
Look up "feline fibrosarcoma". These are vaccine-related tumors, which makes it so much worse. There is a 1 in 10,000 chance of getting such a tumor, but this is my second cat to have one. (The last one had to be put down in 2003.)
We are taking him to a specialist next week who may be able to perform radical surgery on him. It's tearing me and my wife apart because we're so close to our pets.
Good luck on-ramp. I feel for you!