I suppose it’s possible that age of first use could decline if marijuana was regulated, rather than banned. However, please pardon my skepticism at the Marijuana Policy Project’s concern about adolescent marijuana use. I’d guess that some of the factors associated with decreases in tobacco use include increased prices and shifts in cultural attitudes (it’s gone from adult, cool and sophisticated to unhealthy, unsafe and dirty–from celebrated to tolerated) . In terms of cultural attitudes, perceptions of marijuana seem to be moving in the opposite direction, at least in part, due to the efforts of groups like the Marijuana Policy Project.
I don’t begrudge them attacking the irrationality of American drug policy, but I don’t trust their motives. I’m not convinced that “reducing harm” is their primary motivation. They earnestly believe legalization will reduce harm, but legalization is primary their primary goal and I suspect they would not change their position if new evidence emerged to challenge their beliefs (not that I’m predicting new evidence). Along these lines, with their consistent message that it’s relatively harmless, I find it difficult to believe that they are very concerned about teen marijuana use.
One more thought–alcohol is regulated and widely used by teens. It’s possible that, on balance, we’d be better off with different marijuana laws, but this is a pretty weak argument. All policy decisions are about trade-offs. There is no drug policy that will deliver us from all of our drug problems. Beware of people suggesting otherwise.