The knowledgeable young man in the front row of the class looked puzzled as, tentatively, he raised his hand to answer me. His response surprised me. Between us, his bafflement and my surprise measured a gap ¨C cultural, generational ¨C in mutual understanding.

The course was on the history of humans¡¯ perceptions of and relations with other apes. When I asked ¡°What¡¯s a Jacobite?¡±, I was filling in the background to the career of James Burnett, Lord Monboddo, the Enlightenment advocate of the humanity of orang-utans. My student knew that Jacobites were Monophysite Christians in Syria, but, reasonably enough, could not see how they belonged in 18th-century Scotland. It had not occurred to me that my question would prompt him to say so. He felt bound to the truth, even if it seemed nonsensical.

I tried strewing clues, and found, to my surprise, that the class had never heard of Charlie is my Darling or The Skye Boat Song. One of them, a member of the Notre Dame Bagpipe Band, which plays the university¡¯s football team on to the pitch every week, knew the melody of Will Ye No Come Back Again? but not the words or their significance.

¡°The Skye Boat Song¡±, I hazarded, ¡°was recorded by Paul Robeson.¡± I really felt my age when it emerged that no one in the class had heard of him either. ¡°Robeson,¡± I said, ¡°was one of Senator McCarthy¡¯s victims.¡± At last we were all on the same page. But it was a long way back from there, via the persecution of communism in the US in the 1950s, other forms of romantic dissent and the music of Jacobitism, to Lord Monboddo¡¯s opinions on orang?utans.

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After class, I searched the web for videos that might show how modern Scottish revolutionaries ¨C socialist and nationalist ¨C have appropriated the songs of romantic reactionaries. Robeson¡¯s 1961 version of The Skye Boat Song was disappointing. Few basses have ever sung better than he, but in this instance his rendition was inaccurate and emotionally unconvincing. The best recording I found was by the great Kenneth McKellar ¨C lyrical, plaintive, haunted ¨C but I could not show my students the video, for, while the voice, recorded in 1957, was still young, the singer was old and paunchy by the time he was filmed, comically attired in gaudy Highland dress and an inappropriate bearskin. My class would think the effect clownish and ridiculous. I loved Eddi Reader¡¯s tough, flaring, bawdy recording of Charlie, recorded in 2004, but in the film of her singing it, dowdy, ageing, balding folk rockers surround her, and everyone looks awkward. Videos of The Corries display a great array of the Jacobite repertoire, filmed, however, in such a matter-of-fact fashion that it would have taken me too long to explain to the class the connection with modern Scottish nationalism. I abandoned the idea. After all, I reflected sadly, Jacobitism was only indirectly connected with Lord Monboddo¡¯s opinions about apes.

Episodes of classroom misunderstanding multiply as teachers get older. Long ago, when I was young, my students and I shared the same referents, hummed the same music, watched the same cinema, derided the same politicians, admired the same saints and heroes and laughed at the same jokes. Now no one has heard of Bob Hope (even though the great comedian has a room in the Library of Congress devoted to his famous file containing more than half a million jokes), or appreciates my anecdotes about Marilyn Monroe or Adlai Stevenson. In class a few days ago, trying to explain the history of how ambassadorial appointments are made, I referred to Irving Berlin¡¯s brilliant post-war musical comedy Call Me Madam, about an oil-heiress from an Oklahoma dirt-farm who becomes Harry Truman¡¯s envoy to a tinpot European principality. Irving Berlin was a vague name to some of the class, but no one knew the musical, or any of the formerly famous numbers ¨C not even the musically revolutionary duet You¡¯re Just in Love, with its two complementary, simultaneous melodies. The students failed to laugh when I sang them Can You Use Any Money Today? ¨C not even chuckling at my bad voice, let alone the clever lyrics that satirise the Marshall Plan from the US taxpayer¡¯s perspective.

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When I first worked permanently in the US, the deans of Tufts University told me that their best teachers are beyond European retirement ages: typically, in their seventies. I dreaded the UK¡¯s retirement rules, and the thought that old professors could just fade away in honourable senescence was one of my main reasons for crossing the ocean. I have begun to find that a grandfatherly relationship with students confers great benefits of mutual kindness, and calls forth touching efforts from the young people. Failed jokes and antiquated allusions work well, in some ways, in contributing to the cordiality that binds my students and me.

But to communicate successfully in class, I need the gifts conferred by the study of history: of sympathising across chasms of time and culture. I have begun to wonder how long it will be before I start to fade away myself, or topple into the ever-widening gap. Compulsory retirement ages are arbitrary and iniquitous. Wisdom and maturity ¨C which it has taken me a long time to attain ¨C are irreplaceably precious. But they have to include the wisdom and maturity to know when to go.

Felipe Fern¨¢ndez-Armesto is William P. Reynolds professor of history, University of Notre Dame in the US. His latest book, A Foot in the River: How Our Lives Change ¨C and the Limits of Evolution (Oxford University Press), was published this month.