When is my cat too old?

Cats tend to have long life spans. I have had cats live up to 22 years old. I think that your framing of the question, when is my cat too old, indicates that you are not prepared to give a forever home to this cat and don't want to keep it once it is no longer a cute kitten. If that is the case, I strongly encourage you to find a no-kill organization that will take the cat and put it up for adoption.

Yeah

My grandma's cat is twenty and he still walks around, plays, and eats food normally. Sure he's old, but he's still happy.

I think when it hits about the age of 20.