ANNA WAS 15 when she joined her first “street team”. Three years ago she spotted a request on a music website for young people to work promoting bands to other teenagers.

Anna started out doing promotion for a London-based company called Digital Outlook. She e-mailed DJs and wrote to magazines on the bands’ behalf. She received no money for her labour, but she was given rewards, such as T-shirts, gig tickets and CDs.

In the course of her short career, Anna has promoted Avril Lavigne, the Foo Fighters, allSTARS, Hear’say, A-Teens, Enrique Iglesias, Blink 182, and many other groups.

“I let people know I’m a teamer,” Anna tells me. “You feel important saying that.”

Anna is one of many teenagers who market entertainment and products to other teenagers for free. It’s all part of a larger trend, so-called “peer-to-peer” or “viral” marketing (“viral” because it’s delivered by word of mouth — literally or via the internet).

The appeal of viral marketing lies in the fact that it is cheap — and clever. Teenagers are bombarded with conventional advertising and have become resistant to it. Viral marketing, which is more personal, and which comes via their own peer groups, has become the great hope of those trying to sell to the niche teenage market — a niche that spends £900 million a year, according to experts at last year’s Teen Power conference in London.

In the UK, teenagers like Anna sign up with street-team companies, which include Traffic Online, Digital Outlook and the Nottingham-based Promo-Team. These firms work to attract and mobilise groups of teenagers to promote bands online, to video and radio programmes, at gigs, shops, schools, and in clubs and skate parks.

In return, the teenagers (typically aged 14 to 19) receive freebies — T-shirts, concert tickets, stickers and so on. But perhaps the most important reward is an affiliation with the band or product, and a sense of inclusion in a world bigger than themselves.

In the United States, a large number of bands have teenage street teams, who even send out mass mailings to magazines and television programmes urging them to put the stars on their pages and in their shows. The Backstreet Boys, Nelly, Jones Soda and even the environmental body PETA have street teams.

Street teams are just one example of how adolescence has become commercialised. Credit-card companies in the US enlist students to promote their cards at public events, and soft-drinks companies get young bloggers (people who keep weblogs — online diaries), to write about their favourite new drinks on their web diaries. Some American companies have been gathering personal information about children in schools, including their home addresses and exam grades, and selling it to companies targeting the teen market.

In another development, the daughters of American sales reps who work for the cosmetics line Mary Kay have been recruited to sell the brand’s new teen line, Velocity, to their friends at cheerleading parties and bridal fairs. Teenagers routinely “work” for magazines and companies as product consultants or focus-group members. One teenage boy in New York told me that he was rounded up while playing basketball to take part in a focus group and discuss which Converse slogans most made him want to buy the shoe.

Another teenager said of her focus-group experience: “They showed me an ad that was so superficial, the girls emaciated, the copy weird. I showed them how to make them more real.”

The new batch of fashion-led teen magazines, the latest being Teen Vogue, ask their readers to become trend-spotters or, in the case of Teen Vogue, “It Girls”.

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