I read the book when I was 12/13 years old, having seen the film quite some time before that. I loved the film and the book, so obviously it didn't bother me, and I'm a girl. The book is of course written for an adult readership, and it took me a long time, but I would not have deemed it inappropriate. The same year, I read Forrest Gump, which has plenty of sexual content in some parts and (the movie, which is toned down in that regard if I remember correctly) is rated PG-13. It's really all relative. Certain people can handle more than others, and ratings vary between countries. For example, in Canada, it seems our 14A is equivalent to 18A in the US. It's not up to a governing body to decide how old you have to be to watch something, unless one can't be admitted to a theatre or rent it oneself. Parts of the Wizard of Oz scared the crap out of me when I was a kid, but who do you think it's made for?
Review of A Far Off Place
