Is Cambridge University right to enforce a retirement age? I think so – who wants to be a ‘job blocker’? | Mary Beard

Some say it's ageist, and they have a point. But whether in academia or elsewhere, it's only fair for younger colleagues

• Mary Beard is an author, an honorary fellow of Newnham College and former professor of classics at Cambridge University

What is a good" and useful" old age, and how do we ensure it? Ageism is currently one of British culture's biggest muddles. We rightly deplore turfing senior citizens - at some arbitrary retirement age - on to the scrap heap, their allotments, or towards decades of rail travel visiting the grandkids, courtesy of the Senior Railcard. And we love those doughty old ladies of broadcasting (think Joan Bakewell) who turn up from time to time to run rings around some unfortunate young man of 40. But whatever the rules of any profession, no one fancies having their heart bypass carried out by an 85-year-old. Nor do they want to force a scaffolder up 10 floors of dodgy metalwork much past their mid-60s - or watch a presidential debate in which one candidate can't get the facts right and the other can't get out his words.

There's also the inconvenient truth that sometimes, keeping options open for the elderly can prevent,or delay more junior people getting a proper foothold on the career ladder. Forms of discrimination and fairness sometimes clash. You can see that in the argument over Labour's proposed introduction of a retirement age of 80 for the House of Lords. Yes, it can be seen as an ageist gesture that would deprive parliament of some of its most experienced voices. But how do we rate the importance of hearing new people and voices, rather than making them wait until they are equally venerable?

Mary Beard is an author, an honorary fellow of Newnham College and former professor of classics at Cambridge University