(((Treehouse and pet cat))) - I agree with elliemay and I understand how you feel. You can call your vet and tell him/her what's going on and ask if you should bring him in or just let him go through this process if he doesn't seem in pain.

When I younger and living at home we had an 18 yr old cat. He was getting senile and roamed around a lot but he didn't seem to be suffering. One day we woke up and he had died a natural death.

It sounds like it's his time to go and I suspect the only thing the vet would do is euthanize him. If he's suffering then you should go that route but if he's not then he can go in his own time. Usually an animal will seclude themselves like this. I'm sorry this is happening and I think it wonderful he lived this long.