So I'm 19 and I write. Big deal

"The Most Extraordinary Discovery of the Year" (her publisher's words) is a teenager. And boy, does Jenn Crowell get sick of talking about her age. "I can see why it's such an inevitable publicity factor," she says, "but to me I'm just my normal 19-year-old unfascinating self."

But it is more than just her youth and the fact that she has written a pretty good book that make Jenn Crowell interesting. For in the era of confessional memoirs and autobiographical novels, Jenn Crowell has taken the shocking route of Making It All Up.

Necessary Madness, written when she was 17, is no coming-of-age novel but the story of a 30-year-old, Gloria, cruelly widowed by leukaemia, trying to come to terms with the loss of her husband and a visit from her estranged mother. It is the product of Crowell's fertile imagination, allied to the normal human emotions on display in Jaccobus, the small town in Pennsylvania where she grew up.

Even more staggering, at least for the publicists, is the fact that the story is set in London, a city Crowell did not visit until after writing the book. "God forbid an adolescent should transcend her own experience," she says wryly. "The definition of a novel to me is that it's fictional. It's kind of crazy people have forgotten that."

She doesn't think Necessary Madness "is a 17-year-old's novel", and she's right. Her book is more interesting, more complicated than that. It is about love - romantic, maternal - and loss, and includes, for example, an account of a brutal and inexorable descent to death by illness.

Criticism of Necessary Madness does tend to centre on a certain immaturity in the story - as one US reviewer pointed out, Jenn Crowell understands that tragedy does not ennoble, but "can't bring herself to explore the darkest shadows" of Gloria's grief.

Jenn Crowell is slight and, disappointingly, blonde. There is no trace of the punky green hairstyle - "a 19th-birthday present to myself" - that she sported while publicising her book in the US. She is reluctant to proffer stories from her own life that might explain her ability to write about grief, though the deaths when she was a child of her grandfather and an infant brother clearly left a mark. Instead, she offers as an impetus a magazine story about a photographer who recorded her own battle with cancer.

She is also keen to point out that her mother and stepfather have always encouraged her writing, and that they are not the models for Gloria's ghastly parents, both mourning the loss of first love - in his case a woman, in hers the cello.

She writes very well about relationships, particularly those between the child Gloria and her needy father which, rightly, is later described as "emotional incest". Gloria's mother is summed up in a line: "All the skeletons in my mother's closet are tightly zippered in garment bags."

For Crowell, writing does not, it seems, spring from a fashionable need to make sense of her life: she just is a writer and has been from the cradle. "I pretty much wrote before I could even write," she says, as the friend accompanying her book tour sits nearby, stitching a tapestry. "I would dictate to my family." It was the usual stuff, she says, talking animals and the like. "I can never remember not writing."

The family has saved this juvenilia, but Crowell, asked if she has reviewed the material recently, shudders a "No". The one thing she has looked at, she says, is a childhood journal "in which I said, 'If you're not published you're not a real writer'. Pretty cocky for a seven-year-old."

And that is how her life proceeded: at 14 she joined an adult writers' group and started her first year of university at Goucher College in Maryland while simultaneously finishing her last year at school. At college she was taught by Madison Smartt Bell, a novelist who read the manuscript of Necessary Madness and despatched it forthwith to his agent, without mentioning the author's age.

The novel was snapped up: US, British, film and audio rights are estimated to have cost about $800,000, but this almost-million-dollar deal is definitely expected to bring a profit to the publishers. Hodder & Stoughton, Crowell's British publisher, plans a print run of 25,000 hardbacks, which is staggering for a first novel - the norm is to run off 2,000 in hard covers. Hodder is already planning to reprint, again in hardback.

On the subject of her youth, Crowell is blunt. "The thought has crossed my mind that people must have very little faith in youth to be going into ecstasies about me." So what does she want to talk about, if not her age? "Um..." It is clear what she doesn't want to talk about - her sex life, for example. Nor does she mention her extreme short-sightedness due to her premature birth and a shortage of oxygen.

She prefers to discuss theories of literature and feminism, and the process of writing her book; she starts with character and sees plot and structure fall into place as the characters develop in the writing. "I look back at [Necessary Madness] with an incredible amount of fondness," she says. "I don't go 'Oh my God, I should have changed this and that'. It is definitely an extremely hectic, ambitious, ramification-filled way to come out with a first novel. I would probably say, 'Don't try this at home'."

Crowell is not unique in publishing so young - Francoise Sagan wrote Bonjour Tristesse at 17, Rimbaud had written most of his important poetry by 21, the age at which Gore Vidal and Brett Easton Ellis were first published. And in the Nineties, we have been encouraged to read Lara Harte (18), Vanessa Walters (17) and Bidisha (16).

Crowell, a self-possessed intellectual sophisticate who knows (I think) that she has led a rather sheltered life outside the confines of her energetic mind, is also keen to portray herself as "normal" rather than a prodigy. "I can be a normal moody teenager when I want to be," she says firmly - though one does not really think of her behaving as badly as most students.

It is a relief to find that, having learnt much of what she knows of London by watching Absolutely Fabulous, she relates most closely to Edina, rather than Saffy: "I'm really a lazy lush at heart." With all her talk of discipline and focus, and the hard work required to write a novel, I worry that she doesn't have enough fun. Ask what she does other than write and read serious novels, and she replies: "I love music," but then subverts the effect by adding: "I read a lot of feminist theory and I'm really into linguistics."

Necessary Madness, despite its flaws, is an interesting and moving book. None the less, success on a global scale is undoubtedly assured by the book's Unique Selling Proposition. Crowell is working now on her second novel, set in Iceland; she must hope that from now on it is her writing that sells, not her youthn

'Necessary Madness' is published by Hodder & Stoughton, pounds 10