In this age, everything is being categorized and rated for content; video games, movies, etc. However, books should not be included in this rating system. A book is a complete different subject material with all images being created by the reader, instead of by the movie or video game industry, which usually tends to lean towards making most images as graphic as possible to create emotional response in a crowd of movie-goers or in a group of friends playing a game. A book should not be rated in the same way.
Referring to what was previously said, all types of media aim to drive their messages, pictures, and emotions into their audiences to provoke as much of a response as possible in them. Any author that can paint a clear picture of the context of a story for the reader succeeds in what they intend for the audience to understand. One might say a successful author can make a story so it is as if the reader was watching a movie in their mind. With this being said, the content of a story is going to be written so that the reader is going to understand what is going on throughout the book. This includes possible profanity, violence, and any other content that would be deemed as explicit. Children will still be able to gain an image of a story meant for an older audience, and therefore, explicit material would be exposed and interpreted by a minor. A book, like all other types of entertainment, could very well deserve a formal rating system showing the appropriate age group the book is meant to be read by.
You say that children will still be able to gain an image of a story meant for an older audience. However, how would they be able to do this without a previous image in their minds of the actions/objects from an image or movie? Unless a child is already exposed to explicit material, they wouldn't understand what would be going on, and when a child doesn't understand, commonly, they lose interest. Most of the explicit material would not be understandable until a child is grown and has been confronted by their parents about these topics. A child simply would not know enough to understand in the first place. This puts the primary use of warning labels invalid, as children would put down the books without them and have no mental image to deal with.
Children don't have to be exposed specifically by images, being things they see; many other types of media that involve other senses can influence children immensely, such as music, and for the sake of the argument, books. Even hearing adults talk and use vulgar language will provoke children to use that language as well, not knowing what it means.
Music is a huge contributor to children being exposed to and inevitably learning the explicit material they are listening to. Music provoking drugs, sex, violence, and vulgar language gets its message through to children. This of course leaves children prone to curiosity and exploration of mature content. Another important thing to note is that music does in fact have a rating system as well, informing possible listeners that the lyrical content is explicit and not meant for younger audiences. This shows that society realizes that music does have a strong impact on children's minds, even without the use of imagery.
So why wouldn't reading do the same? Books and other reading material can have the same effect that music does and hearing others talk do. Minors can still understand just as much from reading as they do from listening to others and to music.