Charles Aznavour, one of the greatest singer-songwriters France has ever known, sits in a velvet armchair a few days before his 91st birthday, discussing the whiff of ladies’ armpits.

A song on his new album, in which he declares, “I love the smell of your underarms,” worried his Swedish wife of 50 years, but Aznavour knows his audience. If he’s the most successful French crooner in the world – a lyricist who defined the country’s popular culture for decades – it’s precisely because his songs have always been risky.

When Aznavour began writing in the 1940s, sex was something that happened with the light off. It was OK for women singers to howl over their broken hearts, but men didn’t sing about their own emotional despair – and later their dodgy prostates. Aznavour shone a spotlight on masculinity and libido, singing about depression, sex, prejudice and rape. His hits ranged from the 1970s story of a gay transvestite in What Makes a Man, to the once-banned ballad of muggy, post-coital exhaustion, Après l’Amour, and the controversial You’ve Let Yourself Go – the plea of a man whose wife has grown dowdy and fat (“I gaze at you in sheer despair and see your mother standing there”).

Charles Aznavour: ‘It’s a kind of sickness I have, talking about things you’re not supposed to talk about.’ Photograph: Olivier Pirard/Rex Shutterstock

He is unrepentant. “It’s a kind of sickness I have, talking about things you’re not supposed to talk about. I started with homosexuality and I wanted to break every taboo.” The armpit line comes in a new ballad about a blind lover’s sense of smell. “When I wrote a song about the deaf [Quiet Love], I learned sign-language to perform it on stage. On this album, I wanted to describe what it was like for someone non-sighted.” He pauses. “I still don’t know how I’m going to perform it …” In his shows, he takes on various personas with dramatic gestures that resemble a mime act. He’s an actor who sings rather than a Frank Sinatra-style singer who acts.

Aznavour is still composing and performing, he’s written around 1,200 songs and sold more than 100m records in his 70-year career. France worships him as the last living legend of a golden era. Like many popular singers who came to represent the very essence of France – such as Georges Moustaki and, to a certain extent, Edith Piaf herself – Aznavour is shaped by his foreign roots. Born Shahnour Varenagh Aznavourian in Paris to an actor-father and singer-mother who had fled the Armenian genocide, he left school and became a child actor at the age of nine. He survived the German occupation of Paris singing in cabarets, while his parents hid fellow Armenians, Jews, Russians and Communists in their apartment and his father joined the resistance.

Piaf pestered him to have a nose job. Afterwards he presented himself for inspection. 'I preferred you before,' she said

But Aznavour’s path to success was long and torturous. French critics dismissed him as repulsively ugly, too short, with a terrible voice and dubious song titles. It wasn’t until the end of the 50s, a decade after Piaf had taken him on as her songwriter, flatmate and all-round bag-carrier that he finally began to make it. In 1960, he played the shy and haunted piano-player in François Truffaut’s classic New Wave film, Shoot the Piano Player (he went on to act in over 60 films). But his global singing fame was cemented in the 70s with a triumphant crossover into the US and UK – something he puts down to the excellent translation of his lyrics into English. (The bittersweet British No 1, She, is hardly known in France). Britain was seduced by this scrawny Frenchman crooning about painful crushes in a 10-ton accent. “I often say: ‘France is for lyrics, England is for music’,” he muses.

Nowadays, Aznavour is a “dinosaur” – his word – who trades on agelessness. His 51st studio album is out in the UK now and he is working on his 52nd. He loves being sampled by adoring French rappers. He relishes the irony that at 30 he was considered ugly, but past 90 he is now seen as dashing. What it’s like being 91? “I wouldn’t have a clue,” he says, wide-eyed. “I don’t feel 91. I’ve always thought a person must never lose the gaze of a child.” At 5-foot-3, he holds his tiny frame perpetually taut (keeping his shoulders straight is one his secrets of eternal youth). But he’s brutally honest about performing on stage. “I hide nothing from the audience,” he says. He tells them he has an Auto-cue because his memory is fading, and says his mouth ulcers make it hard to sing. He relies on hearing aids. But he loathes what he calls the show-business “cult of youth”.

“More and more men are changing themselves, having surgery, and you can see it on TV, because their dyed-black hair turns blue under the lights,” he says. “I had a problem with my nose, I got it done. I made some white hair that was falling out grow back. But I left my wrinkles where they are. And I look younger than the others because I have never retouched nature’s work.”

Aznavour at the Olympia music hall in Paris in 1971. Photograph: Michel Ginfray/Sygma/Corbis

In fact, it was Edith Piaf cabaret superstar and queen of chanson française, who forced Aznavour to have a nose job 50 years ago. She pestered him for months to fix what she deemed his too-large hooter. He eventually went under the knife, and presented himself for inspection. “I preferred you before,” she said.

There’s a song about Piaf on the new album. It is the first time he has written about her, though they lived together – platonically – for eight years. “We were like cousins. We had this extraordinary complicity. I never had a love affair with her – that’s what saved us.” Why did Piaf, the star, latch on to him, an unknown nine years her junior? “I brought her my youth, my madness, she loved my whole jazzy side.”

I can’t say anything about [Kim Kardashian], because I would anger half the Armenians.

His other main role today is as one of the world’s most famous Armenians. He has finally taken dual Armenian citizenship, is Armenian ambassador to Switzerland and travelled with the French president François Hollande to mark the centenary of the Armenian genocide this year. But France still defines his identity. “I’ve always felt totally French. That really vexed the Armenians in Armenia, but now they’re used to it.” He politely declines to say what he thinks about his challenger as pop culture’s international symbol of the Armenian diaspora: Kim Kardashian. He’s never met her. Does he watch her reality show? “I can’t say anything about it, because I would anger half the Armenians.” He laughs nervously. “I suppose Armenians are quite prudish and don’t like too much nudity …”

Aznavour with Giovanna Ralli in 1961 on the set of the film Horace. Photograph: Gamma-Keystone via Getty Images

A few years ago, he caused shockwaves in France by saying he’d paid backhanders to figures on all sides of the political spectrum after being told he was facing a tax inspection, presumed to have been in the 1970s. A later tax investigation found no irregularity. Decades ago, he left France to live just over the border in Switzerland. “I was never a tax exile,” he is at pains to point out. “I didn’t have a penny when I left.”

The phrase Aznavour probably hates the most is “farewell tour”. He swears he has never uttered the words, and vows to keep performing until he dies.

“You’ve got to learn to leave the table when love is no longer being served,” he once crooned. But with audiences still dishing up a never-ending pot of it, he’s happy to stick around.