I remember reading something like this ages ago and this is all I can remember, it was a book about a teenage girl having an affair with an older man, might have been married I can't quite remember! Hopefully someone can help! Thanks.
There's a novel called 'A Certain Age' by Rebecca Ray which I loved, which sounds similar. A teenage girl with problems at home and school lies about her age and starts seeing an older bloke she meets at a hi-fi shop - Could that be it?