Personally, it made me sad when I saw Feinstein's condition after she returned to the senate. As a human, I had empathy for her.
Having said that, as far as I know, everyone in office is human (with the possible exception of Trump, who I suspect may a shape-shifting space lizard). If a person's subjective desire to stay in office until they die is okay with the rest of us, then there's really no such thing as being too old to be in office, and discussion of the topic appears to be moot.