Labour politicians have criticised David Cameron for what they said were "disgusting and shameful" remarks about long-serving MP Dennis Skinner.
In an answer to a question from the Bolsover MP in the Commons, Mr Cameron advised him "to take his pension".
Mr Skinner, who turned 80 earlier this year, has been an MP since 1970.
Labour MP Toby Perkins accused the PM of age "discrimination" while shadow foreign secretary Douglas Alexander said the remarks were "shameful".
But one Lib Dem commentator has accused Labour MPs of "faux outrage" over the affair.
Mr Skinner and Mr Cameron regularly clash the weekly session of prime minister's questions but Monday's exchange came during a brief debate on the conduct of Jeremy Hunt in response to an urgent question from Labour leader Ed Miliband.
Mr Skinner said the prime minister wanted to keep the culture secretary in his job since while Mr Hunt was "in the firing line then it prevents the bullets hitting him". In response, Mr Cameron said Mr Skinner "has the right, at any time, to take his pension and I advise him to do so".
The PM's response angered Labour MPs, with shadow employment minister Toby Perkins raising a point of order later in the Commons claiming it was not the first time Mr Cameron had declined to ask one of Mr Skinner's questions "on account of the honourable member's age".
Speaking on Monday, Mr Perkins said: "This sort of discrimination would not be accepted against black members or female members, so I have no idea why this House tolerates, on numerous occasions, members coming to that despatch box and refusing to answer his questions.
"This House should be above that sort of thing and I want to know what action to make sure today's disgusting spectacle is the last time we have to see it."
Deputy Speaker Lindsay Hoyle, who was sitting in the chair at the time, said there was "no place" in the Commons chamber for discrimination of any kind and that members should show "respect" to all their colleagues.
Two senior Labour figures also expressed concerns at the tone of the PM's remarks.
Deputy leader Harriet Harman wrote on Twitter that "David Cameron couldn't answer Dennis Skinner question so stooped to ageist abuse - shameful."
And Mr Alexander said Mr Cameron's "abusive answer just now towards Dennis Skinner was unworthy of his office".
Mr Skinner regularly taunts Mr Cameron and other senior Conservatives in the Commons.
He was ordered to leave the Commons Chamber in 2005 after accusing the then shadow chancellor George Osborne of taking drugs.
Lib Dem commentator Stephen Tall accused Labour MPs of "faux outrage" over the prime minister's remarks.
"Dennis Skinner is hardly a shrinking violet and if he dishes it out - and he does - he should be prepared to take it," he wrote on the website Liberal Democrat Voice.
Mr Cameron has come under fire in the past for personal comments he has made in the Commons, urging shadow transport secretary Angela Eagle to "calm down dear" and suggesting Conservative colleague Nadine Dorries was "very frustrated". He also recently urged Tory MP Douglas Carswell to get a sense of humour.
The PM has acknowledged he sometimes gets "distracted" by the amount of background noise in the Commons and that he should "try and tune out the noise and just concentrate on trying to answer the question".
Article from BBC