I'm seventeen and I don't know too many girls who look/act like that. HOWEVER none of the girls at my high school are psychic witches who were abused by their adoptive demon father growing up, so I suppose the point is moot
Joking aside, she generally looks a little more like a 19/20 year old but there are some scenes where I could buy her being my age. The makeup and clothing help to make her look older than she is. As for maturity, she doesn't really act like your typical 16/17 year old, but her situation is a bit unique. Now that I think about it, it's unlikely that she has or ever had any friends her age, let alone girl friends. It's possible that the only acquaintance she had to model her behavior from was Vergil.