Changes in marijuana laws have not been found to have a significant impact on rates of adult use.
"There is little evidence that decriminalization of marijuana use necessarily leads to a substantial increase in marijuana use." - The National Academy of Sciences
In short: legal access ≠ increased use.
When it comes to use by young people, several recent reports have found that in the majority of states that have approved medical marijuana, use among teenagers has actually decreased.
Experts say this is due to a diminished “forbidden fruit” effect and decreased access to marijuana as it moves from the unregulated streets – where there are no age requirements – to inside licensed dispensaries, where you need to be 21 to purchase marijuana.