If you look young but really are not, say you are 25, but look 17, you should still have photos of yourself that represent yourself as a teenager. This means you should look at some teen magazines, like Teen Vogue, or Seventeen and get inspired and do some shoots with the style of a teenager in mind. You should also create two compcards, maybe one showing you at your actual age, with a drink in your hand, a pretty shot of you modeling shoes, or a handbag, or wearing a suit. And then make another compcard that shows you in more of a teenage style, or posing in the style of an Alloy catalog or L.E.I jeans ad.
At the casting as well you will want to look the age you are going for, so if the casting is for a 17 year old, you will want to show up and look like one, since many of the girls there will be actually 17 and you will be competiting against their look.