I feel like I am small for my age?

I am an 18 year old male, but my body looks like I am a 15 year old. I'm 5'11" and I weigh 131 pounds, up from 120 about two years ago. The thing that I want to know is mainly about my frame. I have sort of broad shoulders but my chest looks like, it doesn't exist at all. I am just wondering, is this something I can work on by working out more or is it something I will just have to wait to grow in to? I am a very slow grower already, but I don't feel like I have any more grow left in me.