is Someone Ever Too Old to Hold Elected Office?

A recent opinion piece in the New York Times, Joe Biden Is Too Old to Be President Again,  gave me pause and I realized that  age (as in too old, not too young) is once again going to be a prominent part of the mid-terms and beyond.  In this context, is talking about someone's age (as in, too old to hold a specific office)  ageism, or is it a valid criteria for the ability to do the job dependent on the specific office held? Should the focus be on ability (physical, mental, or both) to discharge the duties of the office, the age of the person, or both? Regardless of anyone's respective position, it's going to be an issue in the campaigns.  I'm preparing for a interesting discussion with my students this fall.