Not a manga fan myself but this is a hopeful sign from one of the larger (largest?) manga publishers:
As part of the celebration of its 10th anniversary Tokyopop is preparing to launch the most comprehensive rating system ever applied to graphic novels. With the help of consultant Michele Gorman (Getting Graphic!: Using Graphic Novels to Promote Literacy with Preteens and Teens and Connecting Young Adults and Libraries) Tokyopop has developed a thorough ratings guide that not only reflects the graphic novel industry as a whole but also provides detailed explanations of its ratings, akin to the video game industry's ESRB (Entertainment Software Ratings Board) ratings.
Tokyopop's new ratings guide will include more than 40 content indicators divided among five age ranges. In Fall 2007, a general set of content indicators in addition to the ratings icon will be printed on the cover of each new series. The new system will provide prospective buyers with far more information about a book's content than the previous system (that Tokyopop pioneered), which simply provided the age rating alone.