Found this interesting:

Half of young people using cannabis suffer side effects such as paranoia and blackouts, a UK survey suggests.  More than 80% of the 727 young people in their teens and early 20s polled by YoungMinds had tried the drug - the vast majority before they were 18.  The charity is calling for urgent research on the effects of cannabis on the developing teenage brain […]

Barbara Herts, YoungMinds chief executive, said […] studies show young people who use cannabis regularly or heavily are at least twice as likely to develop a psychotic mental disorder by young adulthood than those who do not smoke.

So they say that virtually all of the research has been done on adults.  Only thing to really respond to that with is “well no shit!” and move on.
Now, the claim about marijuana use & mental problems here assumes causation where there is actually a gray area.  Think about it for a moment: while there are people who use marijuana (among other things) for purely recreational purposes, there is a significant segment of drug use that could be defined as crude “self-medication” for psychological reasons.  They’re likely to have a revealed mental disorder, but aside form the “smoke weed, go nuts” assumption it could also mean that the ones with precursors to such disorders are more inclined to use it, or that some other condition they aren’t controlling for is involved — possibly even mere genetics.  I’m no expert, but logic alone suggests this isn’t as cut & dried as it looks.

BTW: this is not to say I am ADVOCATING minors use marijuana in the slightest, do not read it as that.  I just hope to remind people to be open minded.