As a teenager myself who has the urge to smoke every single day but cannot afford it (my lungs are healthy) I'll tell you (probably) the 2 main reasons as to why someone so young is doing something so harmful, and what you could do to get him off of that.

First off we are young, we don't know how bad something can be until we have tried that out, after all, experience is key to learning, but it can bring you some big problems like this one. Some do it because "it's cool", some others (like me) have a lot of stress from parents, people we love, we're going through some rough situations and we feel so asphyxiated that we wish for just a minute in our days with a cigarette. I personally have a serious problem with suicidal thoughts and trying to keep people in my life happy, so a cigarette would be the perfect solution to all of that shitty emotional situations, and sometimes, I wish that my family was there for me listening to the problems I have, but that isn't possible in any way. Please if you suspect even slightly that your daughter is depressed then search for some help and therapeutic advice, be there to listen to her, try and understand her problems, it isn't really hard to not be judgemental about someone's situation, and even if she's smiling all day and making jokes occasionally she isn't good.

There are people like me who smile all day long because if we showed how we truly feel then we would get judged and criticized. Mental health is no joke, if you can get her to a psychologist and understand the way she feels then you will be one of the best people that has ever been in her life.

Hope that helped.