Really? You didn't know anyone who smoked before 18? I'm just asking, it's something really odd for me.
But I might be giving the wrong interpretation to that "gasp". It could be that smoking before 18 is unheard of to Elliot, but it could also be that he just wasn't expecting Smokey to dare to do something as illegal as underage smoking*, or it's just a concerned gasp as he's worried about her long-term health.
*which would be odd, for someone who broke into a federal facility because it was more convenient than spending a month as a girl.