I only deal with adults because, come on, let's face it, for every smart, responsible 15 year old there are 27,000 immature, stupid ones. When I was 15, I thought I was the shit, but as an adult I realize I was a complete dumbass. I may be out of touch with teenagers today but I don't think much has changed in the 15 years since I was 15,so I would not trust the majority of 15 year olds who are drug dealers.
I don't see any of this as a moral dilemma, though, just more as common sense.

Being an adult pot smoker in today's society is risky enough without doing business with a ninth grader. It seems like one of the easiest ways in the world to find yourself behind bars that I can think of.