Pop quiz time: when you hear the phrase “best of both worlds”, do you think, “Hmm, interesting concept of dualism, denoting metaphysical binary opposition”? Or do you sing: “You get the beeeest of both worlds, chillin’ out, take it slow, then you rock out the show”?

If the latter, you clearly know that Best of Both Worlds is the theme song from the Hannah Montana television series, and the title of Miley Cyrus/Hannah Montana’s concert film, her first big-screen venture. And you are so going to wish you had been at the Odeon, Leicester Square, on April 23 for the premiere of her second, Hannah Montana: The Movie. The 1,700-seat auditorium was crammed to the rafters with kids aged between 8 and 12 (possibly the most signi- ficant marketing demographic in the world right now, but we’ll come back to that), with a good couple of thousand more clinging to crush barriers outside. Incredibly, they all sat patiently munching popcorn for an hour as the mechanics of a movie premiere clanked into action, watching promos for forthcoming shows on the Disney Channel (again, more later), until a strange noise started at the back. It began as an outbreak of involuntary gasping whimpers, which grew into the hawking cries of circling gulls, until you could hear the words start to form — “Ohmygodohmygodohmygod . . . it’s Miley” — then it burst into a full-throated, genuine, Beatlemania-outflanking scream: “Miley! Miley Cyrus! Miiiiileeeeeey!”

And there she was, stepping carefully onto the stage in a body-hugging silver Hervé Léger dress: 16-year-old Miley Cyrus, aka the sitcom characters Miley Stewart/Hannah Montana, the world’s most successful pop star, TV star and movie star all in one cute-as-a-button Tennessee country-girl package. Indeed, she’s more than the world’s most successful pop star, TV star and movie star combined — she’s on her way to being one of the most successful brands on the planet.

In the sitcom, she is Miley Stewart, a plain Jane high-school student by day; but by night she transforms into the world’s most successful pop star, Hannah Montana, through the simple expedient of putting on a blonde wig. It’s a goofball comedy, with story lines about Hannah selling out by promoting a perfume that makes her gag or trying to conceal a pimple on a huge ad hoarding of her face. Usually, Hannah’s antics lead Miley to let down her schoolfriends, and the basic message is: your friends are more important than being superfamous. The film follows the same pattern. Miley Stewart has started to behave like a diva pop star, so her daddy (played by her real daddy, the singer Billy Ray “Achy Breaky Heart” Cyrus) takes her home to small-town Tennessee to learn her some of them old country ways.

It’s a simple idea, but if you start to crunch the figures on Hannah Montana, you find numbers so gargantuan, they sound like a pub discussion of Britain’s national debt. In 2007, the double album Hannah Montana 2/ Meet Miley Cyrus debuted at No 1 in the Billboard Top 200, then spent 12 consecutive weeks in the top five. The last double album to achieve this feat was Stevie Wonder’s Songs in the Key of Life, more than 30 years ago. Miley’s clothing line was the top seller at Macy’s when it launched in 2006, her video game has sold 1.7m copies in the US, her DVDs have shipped 5m copies and a combination of her first novel and her autobiography (remember, she’s 16) has sold north of 30m copies worldwide. Last year, she was listed in Time magazine’s 100 most influential people in the world, and Forbes ranked her at No 35 of their top-earning celebrities, at $25m-plus. The TV series Hannah Montana had a global audience of 200m in 2008. If Miley’s viewers were a country, they would be the fifth largest population in the world — just ahead of Brazil. And all of them are tweens.

Tweens is marketing buzzspeak for those 8- to 12-year-olds packing out the cinema, and over the past four years, they have become manna from heaven for struggling brands and media companies. (Incredibly, the word tween was invented by JRR Tolkien, alongside slightly less successful creations such as silmarillion and tengwestie, and had something to do with hobbits.) The tweens are a function of the chilling marketing acronym KGOY — kids growing older younger — meaning a 10-year-old who thinks like a teenager.

At the premiere, I was sitting next to Ceylan and Corrie from north London, who began their Miley worship as tweens and are now 14. They had been watching the show avidly since it launched, and our conversation was punctuated with ripples of anticipation as they turned their heads to check their idol wasn’t walking in as we spoke. They love the show because Miley and her buddies aren’t the top dogs in the playground. They’re mess-ups, with crushes on the cool kids. The girls empathised. When all the insecurities of adolescence are being dumped on these youngsters before their hormones have even cut in, no wonder they love the idea of dorky caterpillars who transform into rock-star butterflies when school is out. No wonder they consume their way into an identity. Miley/Hannah is Britney lite, a more palatable version of the fallen-from-grace former Mouseketeer.

Last year, the tween pound in the UK alone was worth roughly £20 billion, and it seems practically recession-proof. You might dump your pet, trade in your car and sell your jewellery to make ends meet, but are you really going to deprive your primary-school children of that essential playground status item? To prove the point, global retail sales of Disney Consumer Products, which handles most of Miley’s merchandise, rose from $400m in 2007 to $2.7 billion in 2008.

Mobile phones are made for tweens; they spend more than $4 billion on computer games a year; and the only magazine sector showing real sales growth targets these readers. A sweet and in many ways deeply charming report by the ad man James Myers in the trade magazine Admap rejoiced in the fact that 30% of parents of tweens make most of their purchasing decisions in consultation with their kids — including, and I love this one, the family car.

Hannah Montana and her Disney Channel cohorts from High School Musical and Camp Rock — such as Zac Efron, Jonas Brothers and the Cheetah Girls — were actually relatively late to the tween party. Adam Sanderson, senior vice president of brand marketing for Disney-ABC networks group, joined the organisation 12 years ago, when it had just shut down the Mickey Mouse Club, the breeding ground for Justin Timberlake, Britney Spears and Christina Aguilera. He watched live concert broadcasts for these stars pull in huge ratings for the channel, but ultimately earn far more for their record labels. Disney then dipped its toe into tween programming with Lizzie McGuire, starring the then unknown Hilary Duff, followed up with That’s So Raven, then hit global pay dirt with Hannah and High School Musical in 2006.

“Although each artist negotiates each contract as they see fit, Miley Cyrus, for instance, has her book, record, film and merchandising deals with various arms of Disney,” Sanderson explains. But it’s clearly Cyrus who’s calling the shots in her career. She’s so precocious, she makes Shirley Temple look like an awkward wallflower who bursts into tears at parties. While the casual observer might look at a sitcom in which father plays father, and daughter plays daughter, and think, “Uh-oh, a vehicle for Billy Ray’s comeback”, it’s actually the other way round.

Disney had the Hannah Montana script back in 2004, and was looking to cast the right kind of lead — a girl-next-door capable of serious lip-gloss action for the Miley-Jekyll/Hannah-Hyde transformation. Cyrus first auditioned for the role when she was 11, but was rejected for being too young and inexperienced. A year later, Disney was still searching, and Miley persisted, even offering to pay her own air fare to Burbank to audition yet again. Eventually, despite her lack of professional acting experience, she nabbed the title role. It was only after she had landed the part that Billy Ray auditioned for the role as her father, Robby. Miley was on the audition panel that helped to decide the casting.

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