Is the evidence available strong enough to suggest that it's the marijuana causing this? Marijuana is often used by those suffering from depression and anxiety as a form of self-medication.
In BC, Canada the province's leading medical doctors have come out in strong support of legalization.
There is no possible way a 12 year old is going to take from five sentences that drugs are safe for 24-7 consumption. In fact, I'd argue that stigma and prohibition are more likely to cause a 12 or 14 year old to try pot than anyone asking if substance abuse might have other underlying causes.