Okay so this is my first constructive post, let's hope I don't sound like an ass nor offend you.
I'm 17 years of age and hang round with quite a few girls aged 16-21. So I should technically know.. Well kind of.
Where to start, with the underneath of her lips it seems to be as if it's being dented inwards, most girls I've hung round with and or have seen don't really have a big indent as that. I'm not sure of the perspective view either, not sure if she's tilting her head down or not. Do you have any picture refferences?
I think they could be of good use to you.
Her eyebrows go to far down, most girls I've seen around my age have small and not so bushy nor dark eyebrows, so try to lighten it up a little maybe? And make it shorter instead of so long. Not sure if you understand what I mean.
Her hair is no how a 17ish year old would really have it, although it really does depend on the teenagers style, Scene, gothic, Chav, Skater ect. Once you figure out what kind of style you want to persue research it througholy.
Scene teenagers would have big and poofy hair with bright and attractive hair, well most of them do anyway.
Gothic, well usually it's straight and black or really dark brown.
Chav, usually short bob, Erm. Like what I mean by bob is like long fringe kind of like Rhiana. Not sure how to spell her name but she has a bob a few times where it's really short but same length from her neck to her fringe? Not sure if I described that well enough or not. Possibilty of just a ponytail or even curly fringe layers and rest of the hair is straight but goes over the shoulders.
Skater chicks from what I've seen in my town wear hat suchs as "beanies" and "flatpeaks" as well do the chavs but the skater chicks usually have straight and quite long hair.
Maybe me telling you what I see as a 17 year old may of be some use.
Your image of the girl also has quite large cheek bones, I'd cut them down a bit to maybe something more softer? Like roundish dependant of the weight of that teenager.
Look for popular female singers like Rhiana, Jessie J, Taylor Swift. Those are artists but try to portray the age of teenager. If you're going for the Scene, Emo look then try looking for The Pretty Wreckless, I'm not suere of the lead singers name but she has a youngface.
The nose seems a bit to wide but that's just my speculation.
Sorry for the wall of text, I just hope this helps you somehow.