Got Twilight’s success in mind? Careful! Plenty of writers crave the expanded audience of a crossover novel, but writing one on purpose is a tough gig. The hitch: It’s virtually impossible to aim one story at a demographic spanning 12 years. Different generations, different life experiences, different sensibilities and sophistication. You must pick one specific target audience and hope the other goes for it. Since young people lack the wisdom and self-reflection that adults gain from experience, you won’t capture many teens with a novel written with a post-college, 25- to 30-year-old narrative sensibility. Your best bet is to write for upper teens (16+) with subjects/themes that can engage adults, too. The narrative should be less self-reflexive and the protagonist less focused on his/her role in the Grand Scheme than an adult would be. Check The Hunger Games: A teen is poised to save her world, yet she’s (understandably) focused on her existence and her love interests for much of the 3-book series. The themes of power, survival, and revolution cross this over to an older audience.
Good clear answer!