Why do I still look like a child at 15?

I turned 15 this year and I look like I'm about 10-12. My body is as straight as a ruler, my chest and backside are pretty much flat, and I just have no curves at all. My face looks even younger than my body, like one time I got a comment from someone on facebook on a picture of mine, saying "I used to look like that... when I was 4." It doesn't help that I'm really short at 5'3 ft. My voice also sounds like it belongs to a little girl. People think I'm lying about my age and nobody believe I'm in high school! I don't really care about being curvy or not, but I don't want to look like a child forever. Why do I still look like one, when all of the girls I know look (and sound) very mature? Will I always be like this?