IT is teatime in Southampton, and outside the city's Mayflower Theatre a small huddle of teenagers are hanging about.

Blue-haired girls and black-clad boys skulk warily on the steps or lean against the theatre wall. In the park opposite, little pockets of similarly clad teens make their way towards the Mayflower in a slow-moving pilgrimage of disaffected suburban youth.

In the pub next door, middle-aged men in Ramones T-shirts, greying Oasis haircuts and khaki jackets are grabbing one last pint before they too make their way to the Mayflower. All of which speaks volumes about the pan-generational appeal of the show that's just about to open.

They haven't come out to watch a gig by reformed rock revivalists or the latest TV talent show sensation. The Mayflower is hosting the opening UK dates for a piece of prime-time musical theatre called American Idiot, and the Green Day hordes are out in force.

Once upon a time, Green Day were cartoon punk pretenders formed in 1987 by a pair of Californian teenagers, Billie Joe Armstrong and Mike Dirnt. By the mid-1990s, Green Day were one of the biggest bands in the world. When they released American Idiot as an album in 2004, the Armstrong-penned rock opera debuted at No 1 in the album charts and won a Grammy for Best Rock Album the following year. One reviewer had already compared Armstrong's writing on Green Day's previous album, Warning, to the music theatre works of Bertolt Brecht and Kurt Weill.

American Idiot's narrative of a suburban boy who flees to the city took Armstrong's dramatic sensibilities even further. It also caught the imagination of Tony Award-winning writer/director Michael Mayer, whose Broadway hits include directing the musical of Spring Awakening.

"It sort of haunted me," says Mayer of the original American Idiot album. "I was a big fan, and listening to the album it very quickly became apparent that what Billie Joe was writing was very powerfully connected with what was going on in America politically and socially.

What also struck me was that the narrative was very simple. It's about a kid from the suburbs trying to find life somewhere else. It's a coming-of-age story. So I thought, this is a musical – there could be a life for this. But I didn't think Green Day would ever go for it."

It was only when asked in an interview with showbiz bible, Variety, about Spring Awakening, what else he thought might work as a musical that he threw American Idiot out there. Mayer's friend, actor turned producer Tom Hulse, picked up on this. Hulse had produced Spring Awakening and offered to do the same with American Idiot.

"I said, oh, sure, knock yourself out. I'm sure Green Day want to do theatre."

They did, and, given the go-ahead to develop the original story by Armstrong, Mayer expanded it to follow three teenagers rather than one, as Johnny, Will and Tunny take on a world that includes drugs, pregnancy, America at war and other things that move the show beyond any notions of jukebox musical status. The result of this previewed at Berkeley Repertory Theatre in 2009, before transferring to Broadway, where Armstrong played the lead role before the show won two Tony awards.

American Idiot's UK tour has drafted in a new American cast, led by Alex Nee as Johnny, Thomas Hettrick as Tunny and Casey O'Farrell as Will, plus Trent Saunders as St Jimmy, the iconic figure who leads them into temptation. On opening night they and the another 16 performers onstage more than rise to the occasion in a high-octane impressionistic and somewhat cynical 21st century take on the American dream.

With a live band on a stage that looks like the ultimate boys' den, despite the banks of TV monitors that punctuates action with contemporary projections alongside furiously well-drilled choreography by Frantic Assembly and Black Watch mainstay Steven Hoggett, American Idiot is a reassuringly old-fashioned piece of Americana.

The runaway slackers trying to find themselves could be straight out of a Jack Kerouac novel, while the reconciliation that comes before an unexpectedly downbeat ending suggests The Deerhunter. The grit of the songs comes alive with a potency many modern musicals could learn from.

At the after-show party, the American Idiot cast are unrecognisably glamorous in their skinny-tied suits and prom-night style dresses that still allow for a certain sassiness. Only when the girls somewhat sweetly start to hand out first night cards to each other does it register exactly how young these guys are.

In the morning, the rain-sodden park beside the Mayflower is again a meeting point for dressed-down youth. Look beyond the hoodies, jeans and trainers and it's clear the little groupings aren't the same pilgrims from yesterday. The American Idiot cast have shaken off their hangovers and glad-rags, and have work to do.

In the corner of the room, Hoggett sits between Hettrick and Saunders, giving notes. In the opposite corner, Mayer is impishly holding court. Sitting with his co-stars, Nee may still be a student, but he understands more than most how much American Idiot might mean to his generation.

"There are so many people in the audience like the people we play," he implores. "So for me, this show is a lot about failure in a way that's never talked about. It's a reality check to say it's OK to mess up sometimes, and we need to connect with each other more to deal with that. I think the show acknowledges we're really trying. It's tough to grow up, and it's tough to be a person, but Green Day have always tapped into that, and with American Idiot, it's saying it's OK to mess up a little bit. It's OK to be different."

American Idiot, Edinburgh Playhouse, October 22-27; Clyde Auditorium, Glasgow, October 29-November 3. Visit www.americanidiotthemusical.com.