Justus Rosenberg is still teaching at age 95

Prof. Justus Rosenberg pulled a tattered, marked-up copy of Voltaire’s “Candide” out of a mushroom-brown soft zippered pouch emblazoned with the old Channel 13 logo. “We have an existential problem,” he said, waving the paperback at a seminar of 10 students at Bard College. “The concept of the El Dorado. What does it mean, the El Dorado?”

A young woman with a Bernie 2016 sticker on her MacBook offered an interpretation. Dr. Rosenberg repeated her answer in his hard-to-place European accent: “The city of God on Earth! The perfect world!” He nodded approvingly, and then continued to teach, without pause, for 120 minutes.

Officially, Dr. Rosenberg, who turned 95 in January, retired from teaching 20 years ago. Retirement didn’t suit him.

“His breadth of historical knowledge is evident in every class,” said Amelia Maggio, 20, as her classmates packed up their things.

The students know that he lived through World War II, that he speaks several languages. And yet he has left that history a bit vague. “He has talked about his experience in the war in Paris, and how he had to flee, but he doesn’t really get into the full details,” said Vikramaditya Joshi, 19, who pressed Dr. Rosenberg to come further out of retirement and be his adviser. ...