I'm not going to touch the whole child-rearing issue, I don't have kids and even if I did, it's none of my business.
What I will say is that every society has rituals for passing in to manhood. Even if they are a little more sublimated than the Peruvian tribe I linked to earlier. Risky behavior-including speeding, fighting, drinking, smoking, sex etc are encouraged as ways to prove maturity and masculinity. We celebrate football despite the fact that every year it ends up crippling and/or killing boys. We let 16-year-olds get behind the wheel, even though the highway is probably the most dangerous place you can be in a stable, modern country.
I'm not advocating any of the behaviors I described, I'm just pointing out that right or wrong, they are a part of growing up. Good kids can do stupid things, I know I did (although smoking weed wasn't one of them). Marijuana, in and of itself, is not the assassin of youth. Kids shouldn't be doing it (no matter what your views on legalization) but doesn't ruin people ex nihilo. Yes, we all know somebody from high school that seemed bright, then was "smart, but hanging with the wrong crowd," and eventually just became a part of the wrong crowd. You could say that about the hard drinkers, the street racing guys (I know one who's no longer with us because of his racing habit), the "stud" who's now a teenage father as well. The drug is just another avenue for risky behavior.
I'm reluctant to get in on this conversation, but a few thoughts on this post. I think where people (people meaning "I" in this context) get hung up on statements like this is where you say, "Risky behavior...are encouraged as ways to prove maturity and masculinity." I wasn't
encouraged to do anything of the sort when I was growing up. That doesn't mean I didn't do them, or still do them, but I hardly try to pass any risky behavior off as an attempt to prove masculinity or maturity.
I did my fair share of stupid things, and I paid for them, in one way or another. That, to me, is part of growing up.
And, for what it's worth, I could care less what a famous athlete does off the court (or out of the pool), as long as it doesn't hurt anybody else or steer kids to a dangerous path. When I read about Phelps toking it up, I thought, "What an idiot," not, "what a tortured soul."