What is it that makes a novel primarily for teenagers, as opposed to anyone else? You might be surprised at the debate this question spawns. I've had many thoughtful discussions on the subject, sometimes with critics who raise the question about my own work, but I've also seen YA novelists denounce  and I use the word advisedly  books as brilliant as Mal Peet's Life: An Exploded Diagram as great in themselves but definitely Not For Teenagers.

Peet's book sits in that hinterland where teenagers themselves reside: one foot in youth and one in the great wide world beyond. Aidan Chambers's Dying to Know You, longlisted for this year's Guardian Children's Fiction prize, is likely to fuel the debate. Its unnamed narrator is a 75-year-old author of books for teenagers who is mourning the recent loss of his wife, who has to consider his sciatica before sitting for any length of time, and is constantly taking emergency urinary breaks in roadside hedges.

He befriends 18-year-old Karl. Taciturn but likeable, Karl has already left school and is working as a plumber. He approaches the narrator because his girlfriend, 16-year-old Fiorella  a fan of the author  has tired of his reticence and demanded he answer a series of questions about himself in "full-dress English". What she doesn't know is that Karl is severely dyslexic, unable to translate his thoughts into written words. For reasons of his own  namely that he hasn't been able to write at all since the death of his wife  the author agrees to help Karl.

Their friendship is as unexpected to the characters as it is to us; at one point Karl has a violent run-in with thugs at the pub who put the worst spin on it. But Chambers is so skilled, so calmly truthful in his writing, that Karl's simple, decent humanity and the narrator's careful concern come across as entirely believable. Not all teenagers are the defiant balls of attitude they are too frequently portrayed as in the media. In fact, most aren't, and none of them are that way all of the time. Most of them are like Karl: cautious, principled, finding their way.

Dying to Know You doesn't stay long in its expected Cyrano de Bergerac groove. Fiorella responds quite badly when she finds out Karl's words aren't his own, and a camping trip she takes with Karl to get closer to him doesn't go the way either of them planned. The story darkens, but never gratuitously, and Chambers is unafraid of frank discussions of sex, depression, the death of a parent, and even serious thoughts of suicide. Chambers himself is 78, a few years older than his novel's narrator, and what emerges is not just a moving, unexpected story of the complexity of teenagers, but also a story of later life, of ageing and loss, and what experience really means.

So is this a book for teenagers? Why on earth not? It features two fully realised, complicated teenagers at its centre, viewed with a clear-eyed compassion by an observer who could have tipped towards the alien but remains fully human. It is perfect for that cloudy expanse between older teenager and younger adult, a novel that doesn't pretend to advise, but merely sees its characters for who they really are. No one appreciates that more than a teenager does.

 Patrick Ness's A Monster Calls (Walker), illustrated by Jim Kay, won both the Carnegie and Greenaway Medals for 2012.