Why don't books have age restrictions/guidance certificates like movies?

I never thought about this before as I didn't have an interest in reading novels, but I've started and wondered why no age restrictions. I read "P.S. I Love You". I noticed the book had the F-word a number of times and descriptions of drunk night out. This was made into a movie; certificate... show more I never thought about this before as I didn't have an interest in reading novels, but I've started and wondered why no age restrictions.

I read "P.S. I Love You". I noticed the book had the F-word a number of times and descriptions of drunk night out. This was made into a movie; certificate 12 (UK). I've just started reading "The Notebook", also made into a movie, certificate 12. In this book, there was a discription of a sex scene which I found more discriptive than the scene in the movie suggests. I checked & didn't see any guidance for age and nothing to suggest that the shop keeper or parent to decide whether age appropriate. If a 10-year old attempted to buy "The Notebook" novel, they would be able to buy it without being questioned, but if the same child attempted to by the DVD, an "age check" warning would appear.

Why not age certs on books? The mind is powerful and can easily create images as discribed in books. Do you think books should have age restrictions?
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