Age is a dirty word in the Collins household. Not because Dame Joan’s scared of her 90 years. Or because her beloved fifth husband is a few decades her junior. But because she’s always believed the secret to ageing well is to…well, ignore it as well as you can.

“Oh! We don’t mention that,” she laughs when asked about The Birthday. “I’m 60… plus however many years!”

It might explain why, having reached that birthday milestone in May, Dame Joan still has a work and social diary to rival Alexis Colby in her prime.

There’s the new book – her 19th – Behind The Shoulder Pads, an upcoming tour of her new one-woman show of the same name, a new partnership with Marks & Spencer food, a passion project about Wallis Simpson (which may or may not go down well with her Royal friends), and, more imminently, a host of shopping to do before guests arrive for her regular poker game.

It’s quite the full dance card for a screen icon who could once count James Dean, Gene Kelly and Marlon Brando as pals, and who has known Liza Minnelli since she was a babe in Judy Garland’s arms.

“There’s far too much fuss about ageing,” explains Dame Joan. “You see people in their 90s and 100s doing amazing things – running and jumping on Instagram – there’s no reason to fall apart. I do my stretching exercises each morning and I have a trainer on Zoom three times a week.”

Her routine, she adds, includes “standing on one leg” for “almost a minute” – an activity that would defeat many half her age. However, one might suggest the real reason for Dame Joan’s youthful joie de vivre has less to do with her balancing skills and more to do with the stability and happiness she’s been enjoying since meeting and marrying her “best friend” – hubby number five Percy Gibson, 58.

The pair celebrated their 20th-anniversary last year – a bona fide slap in the face to the one-time critics of their 32-year age gap, and perhaps the reason Dame Joan is suddenly ready to open up about their relationship in a way she never has before.

“It is fifth time lucky,” says Dame Joan, over Zoom, from their grand central London flat. “I’ve been feeling that for years. He’s a wonderful, kind, truly good man. He’s my – what’s the thing he always says? – my accomplice, yes, and my best friend. Plus he’s extremely handsome and he just gets better and better.”

We’re audio-only – Dame Joan has just got out of her mid-morning bath – but you can tell she’s wearing that famous coy smile of hers. “Everyone says how lucky I am,” she adds. “And how good he’s looking right now.”

The pair first met in 2000 when Dame Joan was 67 and touring the US in the play Love Letters.

Percy – whose father was Peruvian and mother Scottish – was her 36-year-old divorced producer. They bonded over otherwise lonely dinners on the road and soon, for one of the first times in her life, Dame Joan fell for the nice guy.

When a last-minute change of plans saved Percy from being near the World Trade Center on 9/11, the lovebirds decided life was too short not to marry – and wed at Claridge’s hotel in Mayfair, central London in 2002, and recreated the day for their anniversary last year. So, has Dame Joan finally found a man to treat her like the queen she is?

“No, not at all. It can’t work like that,” she tells me. “A relationship is about two people both being there to help and support each other.”

It’s not what I was expecting. Although…“Every morning before he goes for a run he brings me a coffee in bed while I read all the papers,” she adds. “And he leaves me little notes under the kettle.”

So what is the secret to this marriage? (Dame Joan once refused to be Liza Minnelli’s maid of honour, saying: “I’m always the bride, never the bridesmaid”, below.)

“I think the difference was, Percy and I became really good friends first,” she says. “So we had this connection. We really knew each other. And separate bathrooms is key! Although we are very lucky. I know that’s not something everyone can have.”

Her point is, that they give each other space.

“He has his office area and I have mine. I’ll spend time in mine writing or phoning my friends. And he’ll be working, looking after our three properties,” she explains – they have homes in London, St Tropez and LA.

“Then we’ll do something together. Like any couple, we have our ups and downs. And we do occasionally argue – it’s normally when we’re working together.”

That bodes well for the next few months: he’s directing her forthcoming UK tour.

Yet you only have to read her new memoir to realise their disagreements are few and fleeting. The pair give “his and her” versions of how they met and their wedding day, and while “some recollections may vary” – as Dame Joan’s idol, the late Queen, would say – they’re clearly besotted with each other. Take this snippet from Percy’s wedding speech: “Joan is my accomplice, my comrade, my confessor, my confidante. My fearless leader and my most loyal supporter, my very best friend.”

Percy’s Peruvian heritage also gives him a refreshing attitude to the age gap. “The age difference didn’t matter to him,” Dame Joan explains. “In Peru, if a woman is attractive, she’s attractive. They don’t think about it.”

Having endured years of ribbing about her “toyboy”, Dame Joan seems pleased attitudes are changing. Even if it’s just that older men are also getting “more of a hard time” lately. We’re discussing her peers Robert De Niro, 80 (whose girlfriend is 45) and Al Pacino, 83 (whose on/off girlfriend is 29).

Both actors have something extra on their plate that Dame Joan does not – a newborn. The thought of having to deal with late-night feeds and nappies again causes much mirth to the mum-of-three and four-time grandma.

“Don’t get me wrong. I adore my baby granddaughter,” she says, of her son’s daughter, 18-month-old Deia. “But a couple of hours with her and that’s enough for me. Not that the men do much of the work, mind you! They do the cooing and that’s it!”

Before Percy, Dame Joan’s love life had been complicated. She was just 19 when she married her one-time heartthrob, Irish actor Maxwell Reed, after he date-raped her on their first night together.

They split after three years and she got engaged to Warren Beatty, only to have to have a then-illegal abortion – she is appalled many states are making them illegal again.

That relationship ended because of Beatty’s infidelity, as did her following eight-year marriage to the late actor-singer-songwriter Anthony Newley – the father of her daughter Tara Newley (59, and a writer and producer) and son Sacha (58 and an artist).

Then in 1972 she married film producer Ron Kass, with whom she had Katy, now 51. By 1983 he had left her broke and was using drugs – something she blames on his cocaine-snorting film protege, and their one-time lodger, Dodi Fayed.

Next came Swedish musician Pete Holm. They split after two years in 1987 and she has no idea, and even less wish to know, what happened to him.

Dame Joan was with The Grand Budapest Hotel actor Robin Hurlstone – who actively hated her family, including best-selling author sister Jackie – when the actress began an affair with Percy. It led to a very messy break-up but one that was worth it. “[Percy] was the best lover I’d ever had,” she raves in her book.

Since then she and Percy have had a few dramas – a fire and a water leak in their London flat, escaping wildfires in the south of France, Dame Joan collapsing in LA (her corset was too tight and her meals too light, doctors said), and three bouts of Covid.

But, for the most part, the only tumultuous love lives that now keep them up at night are the ones they’re binge-watching on TV. So what are they hooked on?

Well, what else… “A lot of TV is so boring nowadays,” says Dame Joan. “Nothing really rocks my boat. So Percy found a box set of Dynasty.

“Back then, we didn’t have videos, so I haven’t seen about three-quarters of them. Percy will be asking what happens next with Blake and Alexis – and I have no idea! I can certainly see why it was as popular as it was.”

Watching her as the formidable Alexis, Percy must also be pinching himself about how lucky he is. No matter how many candles are on her birthday cake.

• Behind The Shoulder Pads: Tales I Tell My Friends by Joan Collins (Orion, £22) is published on Thursday. To pre-order, visit expressbookshop.com or call Express Bookshop on 020 3176 3832. Free UK P&P on online orders over £25. Dame Joan’s Behind the Shoulder Pads tour runs October 1 to 24. For tickets and information, visit amickproductions.co.uk/dame-joan-collins