Prime Minister David Cameron has been accused of ageism after branding a veteran Labour MP a "dinosaur".
He told Dennis Skinner people should not visit the Natural History Museum to see the ancient beasts, but "come to the House of Commons" instead.
Mr Skinner - who has often tangled with the PM - gave a shrug, but fellow Labour MP Paul Flynn called the comments "completely unacceptable".
Former Deputy PM Lord Prescott also defended his colleague, an ex-flatmate.
The exchanges came during prime minister's questions, where left-winger Mr Skinner asked whether Mr Cameron would appear before Lord Justice Leveson's inquiry into media standards, given that he had once employed former News of the World editor Andy Coulson as a press adviser.
Mr Cameron replied that he would be "delighted" to do so, and added: "It's good to see the honourable gentleman on such good form.
"I often say to my children 'No need to go to the Natural History Museum to see a dinosaur, come to the House of Commons at about half past twelve'."
Mr Flynn later raised a point of order, asking if it was appropriate for MPs - including 79-year-old Mr Skinner - to be criticised on the basis of their age.
He said: "One of the areas of parliamentary life that has improved in recent years is it is now completely unacceptable for one Member to criticise another Member on the basis of gender, on the basis of race, ethnicity or disability.
"The most under-represented group in this Parliament are the septuagenarians. Today we heard what I believe many of us (believed) was a gratuitous and entirely offensive insult to a greatly respected honourable Member made entirely because of his age."
Another Labour MP, Brian Donohoe, said the prime minister "should come back to this place and apologise to Dennis Skinner".
But Conservative Mark Pritchard said: "I wonder whether this House of Commons would be better with more or less humour?"
Replying to the points of order, Speaker John Bercow said: "I'm always in favour of humour but just as beauty is in the eye of the beholder, humour is a matter of subjective judgment.
"Sometimes people are funny. Sometimes they think they are funny. Sometimes they think they are funny deliberately when they are not. Sometimes they don't realise they are funny when they are."
Last year, Mr Cameron was criticised for telling Labour shadow minister Angela Eagle to "calm down, dear" - the slogan used by film director Michael Winner in adverts for car insurance.
Labour leader Ed Miliband later likened Mr Cameron to Flashman, the bully from the novel Tom Brown's Schooldays.
Lord Prescott continued the theme on Wednesday, when he wrote on Twitter: "On my old flatmate Dennis Skinner being called a dinosaur by Cameron. Better a dinosaur than a Flashman!"
Mr Skinner, an MP since 1970, celebrates his 80th birthday next month.
Article from BBC News