Just a lot of personal experiences. Had way too many friends smoke a lot of weed in high school and now have ruined and squandered their futures away by doing nothing but living with parents and smoking weed every chance they get. Some of them were really smart too. There aren't really a lot of long term studies on marijuana and its effects, but my personal hypothesis from what ive seen is that kids who do it young, particularly in high school, kind of become teenagers forever. By that's just personal experience. I'm sure there are exceptions to it, and adults who start it as adults don't have as many issues. Having nothing against it and those who smoke it, but if my kids do it their asses are in serious trouble and I would never want to experiment with it. As someone who can get addicted to just about anything, why risk it?