I don't think I've ever worked with anyone who understands their audience more than Tavi. Here's a new interview where she describes who Rookie is for:
I keep just describing it as a "website for teenage girls," because I like to think it's not too niche, and I don't want to alienate anyone by saying it's for alternative girls or artsy girls or anything. At the same time, I mean, we don't speak for every girl, but we try to encourage girls to speak for themselves. Mostly we just try to avoid being condescending or making anyone feel like there's something wrong with them that they should be worrying about if they're not already. Or like we're teaching anyone how to be cool. I want people to know that they're already cool. Whatever they're into, that is enough.