The Beatle, who had a similar experience growing up in Liverpool, said it’s “scary” that knife crime is so rampant after his grandson had his phone stolen in the traumatic attack.
“One of my grandkids, one of my older grandkids, was mugged and got his phone taken,” he told The Times.
“He was saying the worst thing was that he should have just thumped the guy, he came back and felt a coward.
“I said: ‘No, no, no, no! The guy had a knife and you don’t know, the guy might be able to use that knife.’ So it is scary these days.”
Sir Paul, 77, was able to relate to his grandson’s experience because he was mugged at around the same age in Liverpool, before the Beatles hit global fame.
“When I was a kid it was four guys and they nicked my watch. I was of a similar age,” he went on.
“I just happened to be on my own, bigger kids came along and it was the same feeling.
“[I thought at the time] ‘I have got to learn karate and be a black belt — and then I’ll get ’em!’ It was the worst thing.”
The singer didn’t name which of his eight grandchildren was on the receiving end of the knife threat.
It comes after Elizabeth Hurley called for an end to knife crime after her nephew Miles was stabbed in March 2018 during a row with thugs in the street.
Miles was hospitalised with severe stab wounds and needed blood transfusions when he lost six pints of blood, with the blade narrowly missing a major artery.
“I had a guardian angel that night… The doctors thought I’d perforated my liver and clipped one of my major arteries,” Miles later told the Mail on Sunday.
“But everything they feared could have happened, didn’t. They said I was stabbed twice, in the same place. That the knife didn’t hit my spinal cord or arteries all came down to angles and millimetres.”
His aunt Liz, who flew home from the States to be by her nephew’s side, was said to be devastated after police found the DNA left at the scene in London’s Nine Elms was not of a “sufficient level” to find a match.
The actress called for fans to join her in making the streets of London safer when she shared an update of Miles’ condition on Instagram .
She added: “We need to make our streets safer and I want to be part of the movement to achieve that. I hope you will join me.”