The most challenging thing for me is making sure my characters sound like teens. I'm not a teenager (I know, shocker), but I was one in the 80s. I'd rather my 16-year-old characters not wear leg warmers and listen to Duran Duran. Having to remember kids now live on their cell phones and email is a way of life is hard to remember. Plus, current slang. Like, totally! Not!! (<-- not current). However, teens today face the same core issues we did twenty years ago, self-acceptance, popularity, falling in love for the first time, etc., which is what I focus on (plus add in some demons and curses here and there). All the rest of it is window dressing. :)