How can I look my age, I'm almost eighteen but I look like I'm told I look twelve or fifteen?

I think you have a young looking face and figure naturally and your 'look' is kind of young.I think a more sophisticated hair cut(like a polished bob) and colour(a rich medium brown) would help you to look more your age and more subtle make up.Also maybe wear less casual clothes and try and make your outfits look more put together.