It seems redundant to only have characters whose ethnicity, sexuality or disability is relevant to a storyline. After all, they're irrelevant in real life, or at least they ought to be. The Mirrorist recognises that these aspects of life become prevelant throughout our teens because this is when we begin to comprehend their relevance to our lives. That makes them young adult issues, and it is pointless and foolish to avoid confronting them when catering for a teen audience.