I'll counter with the example of Bernie Sanders, who is 82 years old, still sharp as a tack, and arguably the furthest left person in the federal government. Jimmy Carter is 99 years old, and while I know we're not hearing much from him anymore, he's been an incredible force for good well into his nineties. (Yes, I know he hasn't held elected office since early 1981, but he damned well could have, and done it well.)
Yes, Feinstein should have retired a very long time ago, not because of her age, but because of her mental decline.