It's anecdotal - you have no way to know what would happen otherwise. I have seen plenty of young people smoke weed in their teens while gradually stoppping later and leading completely 'normal' lives, while a few didn't stop and went to heavy polydrug abuse etc. What exactly does that say about weed? Not much I'm afraid.
Similarly, I know of several persons that went schizophrenic after car accidents, but were the car accidents the cause?
Not suggesting people should smoke weed when they're young etc, but there's a reason that we do these gigantic, extremely complex, extremely failure-prone things called clinical trials to actually ascertain the effects of drugs in the body.
It's because we've found over and over and over and over and over that the "I know N people who X then Y" claims were wrong.
For example: Why did those two people start smoking weed so young? Were their lives, families, and personalities otherwise the same as everyone else? Probably not!
Similarly, I know of several persons that went schizophrenic after car accidents, but were the car accidents the cause?