There is significantly more evidence that adolescent cannabis use is associated with psychosis. Though many studies are purely correlational, and because it is unethical/impractical to randomize to cannabis use so we can never get the true randomized tests, there are multiple cohort studies like this that clearly lean us in the direction of **cannabis causing** psychosis.
Bottom line, with our current state of knowledge, early cannabis use isn't a great idea with youth. Later seems better and less seems better. 1-2% of the population will develop psychosis, so you could be exposing yourself to a 1/20 chance of developing psychosis with early cannabis use.
I'm not sure how we'll ever establish cause. But it's ok to take the lean this direction, espscially as there is almost zero evidence of benefit for youth. If benefits emerge, then the weight of our association/cohort knowledge will need to be weighed against it.
It'd be great to put some measure of social isolation out there to see if that is an intermediary variable. Having worked with a great deal of young adult manic/psychosis patients...there often is an associated social isolation component to their story in addition to daily marijuana use.
In other words Does early onset use isolate one from their peers which contributes to social isolation or is it the substance itself that is driving psychotic symptoms towards expression and subsequently social isolation due to reduced reality testing ability ?
Nevertheless, think the research is still fairly suggestive of the recommendation family history of psychotic illness : don't smoke weed until you are 21+ years of age.