Authority is important in advice sites, esp. in fashion, I think. However, there are three aspects of this particular demographic that I think would make the job easier: (1) they are by their nature more or less anti-authority and anti-establishment, (2) AFAIK, there are
no sites that do this sort of thing, and most importantly (3) we are
much more connected (except, perhaps, for teenage girls) so word of mouth is very powerful.
The only thing that sort of comes close in fashion are sites such as Threadless, betabrand, etc., mostly geeky t-shirts. But no site for how to do the whole look.
What you are proposing, I think, is to take the idea even further, give advice on relationships, dealings with pointy haired bosses and so on. I think that would be great.