WHEN the great Pop Fan World War breaks out - Mods vs Rockers, Blurites vs Oasisians - I'm going to be signing up with the Hanson fans. They are terrifying. On Tuesday evening, Wembley Arena was engulfed by a raging ocean of female hormones, ravaged by a marauding army of pubescent girls in crop tops, with braces on their teeth, cardboard signs in their hands, and "Tay" and "Zac" daubed Pictishly on their faces. They're at a dangerous age: young enough to scream themselves in half at any provocation, and old enough to be provoked by the sight of a roadie wearing shorts; young enough to have the nuclear excitability of nursery schoolgirls, but old enough to ensure that if they stampede, innocent bystanders would be crushed.

I wouldn't fancy Hanson's chances in a stage invasion. The boys who fire the blood of this frenzed mob are three polite blond brothers from Oklahoma, and like their fans, they are balanced between childhood and young-adulthood. Lead singer and keyboard player is Taylor, 15: junior Kurt Cobain looks, any-aged Michael Jackson voice. His big brother is Isaac, 17, guitarist and walking growth-spurt. Finally, there is 12-year-old Zac, the drummer, who looks like Macaulay Culkin did in the days when people went to see his films.

The triple-whammy novelty value - they're young, they're brothers, they perform their own material - would be enough to propel them to stardom by itself, but Hanson wouldn't be filling Wembley Arena if they didn't have at least one infuriatingly catchy song. They've got at least two. "Where's the Love" and "MMMBop" sound as if they weren't written, but fell fully formed from the pop-classic tree. They're so fun and clever that they expose Another Level, 911, MN8 and all the other Take That-wannabes - who could swap members every other week without anyone noticing - as the graceless, faceless automatons they are. And if you don't believe me, judge Hanson by the company they keep. The brothers' breakthrough album, last year's Middle of Nowhere (Mercury), was put together by Steve Lironi and the Dust Brothers, producers of Black Grape and Beck respectively. (I call it their breakthrough album because, unbelievably, Hanson had two previous LPs. On the first one, Zac's vocals were recorded via an ultrasound scanner.) Hanson are, strictly metaphorically, head and shoulders above the competition.

After an opening medley of "Gimme Some Loving" and "Shake a Tail Feather", Taylor asks if we're ready to rock. And, my goodness, rock is what they do: Hanson's show appears to be designed as an indie audience training ground. Yes, the boys look cute and play cute songs, but the rites of the teeny concert have been replaced, piece by piece, by the rites of the grown-up gig. Any sort of choreographed dancing is out. Pyrotechnics and video screens are verboten. There is just a band, with instruments, playing songs. There's not a lot of talking to the audience between those songs either, and what there is is usually mumbled. There is a segment in the middle in which the trio sit in a huddle and play some acoustic songs, and for the rest of the time the band are noisier and markedly less polished than they are on their records (Taylor's voice has sunk an octave since Middle of Nowhere was recorded, so some of the harmonies have vanished). Get used to it, Hanson fans. One day, all the gigs you go to will be like this.

The Zacchanalians didn't seem too downhearted by this prospect. The show lacked the pure pizzazz of an all-dancing, all-teasing boy-band spectacular, and any praise for Hanson as a kickin' rock'n'roll combo has to be followed by the qualification "for their age". But they did succeed in mixing some of the bubblegum appeal of the former genre with some of the visceral thrills of the latter. The biggest surprise was to see that although the brothers' sound was beefed up by a bassist, an extra keyboard player and an extra guitarist, there was no back-up drummer. Zac is a startlingly fast and muscular player (for his age).

And unlike almost any other group with a mid-teen fan base, Hanson have a future: they have the natural talent to keep going when they're old enough to vote. On Tuesday, Isaac set out to prove that there's more to them than fizzy pop hits by putting his heart into a sensitive solo piano ballad, "More Than Anything". The Zacchanalians, ignoring his plea for quiet, yelled their lungs out as deafeningly as they did during every other second of the show. So, that's an introduction to another grown- up gig ritual: the bit when the band get thoroughly sick of being screamed at and demand to be taken seriously as mature artists.