From what I can see, Cynthia Harris' last role on the New York stage was in Charles Busch's "The Tribute Artist" back in 2014. It played at the 59E59th Street Theater. Harris played Adriana, a wealthy elderly eccentric, who suddenly dies and her "tribute artist" cross-dressing friend Jimmy (played by Busch) steals her identity (kind of like a benign gay male version of Sante Kimes). Complications ensue as well as unexpected family bonds.
Cynthia Harris was only in the first half of the first act but made a great impression creating a vivid, memorable character. She would have been about 80 at the time.