Marijuana criminalization has failed to keep that drug away from young people; they have been reporting since well before any state had legalized that they could get marijuana almost as easily as cigarettes or beer, although the latter two are much more widespread among adults. The available evidence indicates that the best way of keeping a drug away from young people is to legalize it for adults - which gives its sellers an economic incentive to confine their sales to adults, namely the risk of losing their legal adult market.
or for those who tend toward various mental illnesses
So we should ban for ALL adults whatever is bad for those susceptible to mental illness? Including alcohol?
Marijuana almost always causes depersonalization and derealization disorders. Sometimes it has lasting effect continuing in sober condition. All frequent users are demented. I can easily see a pothead.