Following legalization, the rate of adolescent marijuana use in Colorado has fallen to its lowest level in nearly a decade, according to new federal survey data. State-level numbers from the National Survey on Drug Use and Health show that a little more than 9 percent of Colorado teens age 12 to 17 used marijuana monthly in 2015 and 2016, a statistically significant drop from the prior period. That's the lowest rate of monthly marijuana use in the state since 2007 and 2008. And it's not just marijuana: Rates of teen alcohol, tobacco and heroin use are down sharply in the state, as well.