food for thought

Films have a classification, like PG or 15, according to its content such as swearing, violence, sex, drug use, etc. This makes perfect sense. But what about having a similiar rating for books? The book I'm reading just now, 'Miss Smilla's Feeling For Snow', was made into a film and was rated 15, but there is nothing stopping someone under 15 from buying the book. Is there anything stopping a 12 or 13 year old from going into Waterstones or Borders and buying something from the erotic section? Even a book like Trainspotting would be unsuitable for someone of that age although maybe some would argue that Trainspotting would be educational in teaching young adults about the dangers of drugs. Maybe this is something that should be looked into.