“Quite simply, the best singer in the world”. It was Dame Cleo Laine, of whom Derek Jewell spoke – and as music critic for the Sunday Times for 23 years, it’s fair to say that he knew what he was talking about.

We were at the Southbank Centre for a much-anticipated evening of music and chat with the legendary (and I don’t use that word lightly) Dame Cleo. Jude Kelly, Southbank’s artistic director, kicked off proceedings by welcoming us to ‘(B)old’, a new festival profiling artists aged 65 and over who are established in their practice and recognised nationally or internationally for their work. “Age has a certain radiance”, Kelly stated – “and (B)old is all about asking older people to share their creativity with us”.

And who better to share their creativity than Dame Cleo Laine? The accolades speak for themselves: Dame Cleo, whose vocal range spans almost four octaves, is the only female artist to have received Grammy nominations in its jazz, popular music and classical music categories. Throughout her career, she has worked with countless high-profile musicians: Ray Charles, Frank Sinatra, Nigel Kennedy and John Williams to name just a few.

Now 90 years old, Dame Cleo continues to tour and to perform and tonight’s packed auditorium was a testament to her enduring popularity. She was joined on stage by her band – which includes her son, Alec, a bassist – and by her daughter, Jacqui Dankworth, who conducted the interview.

What a life this nonagenarian has had. Born in 1927 to unmarried parents (then deemed socially unacceptable), Dame Cleo was the daughter of a white mother, Minnie, and a black father, Alexander – who “sang constantly”. Alexander, according to Dame Cleo, was “quite a character”; unemployed during the Depression, he took on all sorts of jobs to make ends meet, including operating an ice cream van, and busking. Alexander’s grandmother had been born into slavery and his father escaped a similar fate by fleeing Jamaica for England; he would go on to serve in the armed forces during World War One.

For her part, Minnie, a “revolutionary woman”, kept boarding houses for the Irish community. Dame Cleo remembers her early life, growing up in Southall, as “full of music and merriment”. There was never any doubt that she would forge a career in music, although she was in her twenties before she became a professional singer, having auditioned successfully for the Dankworth Seven. Dame Cleo remembers the experience well: “It was the height of summer and boiling hot, but I borrowed my sister’s fur coat because I’d read somewhere that’s what you should wear to an audition.”

The Dankworth Seven were impressed. “She’s got something”, commented their leader and renowned jazz composer John Dankworth – who Cleo would go on to marry, after romance blossomed on a tour of Germany.

Cleo performed with the band until 1958, when she left to pursue a solo career and theatrical work, starting with ‘Flesh to a Tiger’ at the Royal Court. Her favourite stage role, she told us, was that of Julie in ‘Showboat’: the role which launched Cleo’s career in America. There, she toured with ‘Into the Woods’ and was nominated for a Tony for ‘The Mystery of Edwin Drood’.

I particularly enjoyed Dame Cleo’s reminiscences about working with Ray Charles (“I had to keep nudging him, because he hadn’t learned his words or the music”) and Mel Tormé. The latter, she informed us, was “known for being tough and difficult to work with, but I seemed to have a calming influence on him”. There followed a heart-warming duet by Cleo and Jacqui of ‘I Thought About You’, which Cleo previously recorded with Mel.

She and John Dankworth toured for many years and often took Jacqui and Alec with them – indeed, Jacqui recounted wryly, it was her job to iron Cleo’s costumes – some of which they had brought along tonight, to show us: a collection of gorgeous dresses by Ossie Clark, Zandra Rhodes and Bill Gibb.

The last song Dame Cleo performed for us was ‘Tea for Two’ – which holds special memories as she and John used to wrangle over the pronunciation of certain of its lyrics. Sung against a backdrop of photos of the couple with Jacqui and Alec, it was an emotional ending: Dame Cleo looked genuinely moved by our standing ovation.

As a celebration of “older” people, tonight could not be bettered: Dame Cleo Laine is living proof that age should be celebrated, not disdained. Tomorrow, I will return to the Southbank Centre for another instalment of ‘(B)old’: this time, to hear children’s author and WWII survivor Judith Kerr speak. I cannot wait.