According to the most recent numbers, the rate of past-month marijuana use among 12-to-17-year-olds was a bit lower in 2016-17, after three years of state-licensed sales (and four years of legal home cultivation in Colorado), than it was in 2011-12.
I have to say I am rather surprised by these numbers. I was never comfortable with assurances that legalization would not lead to more underage use. While it’s true that licensed marijuana merchants, unlike black-market pot dealers, actually card their customers (and anyone who has bought marijuana in Colorado or Washington can testify that the stores take that responsibility seriously), it seemed to me that leakage from the adult market might very well make pot more readily available to teenagers, resulting in more cannabis consumption. So far it hasn’t happened, maybe partly because legalization made marijuana less appealing to adolescents—a reversal of the “forbidden fruit” effect.