Apparently teenagers aren't intelligent enough to be able to intake material more mature than early Mary-Kate and Ashley movies without becoming engrossed and destroyed by it.
Also, books were never dark until now. Even V.C. Andrews' plethora of novels and Tess of the D'urbervilles existed without any disturbing content. As well, in the past, teenagers (who would have had children and a family to care for before most of today's teens can drive a car), were not able to pick up any books beyond fairytales (which were also not disturbing and did not include death, curses, or the slicing of toes to fit into glass slippers).
That is all.