The Washington Post reports:
Rates of marijuana use among Colorado’s teenagers are essentially unchanged in the years since the state’s voters legalized marijuana in 2012, new survey data from the Colorado Department of Public Health and Environment shows.
In 2015, 21 percent of Colorado youths had used marijuana in the past 30 days. That rate is slightly lower than the national average and down slightly from the 25 percent who used marijuana in 2009, before legalization. The survey was based on a random sample of 17,000 middle and high school students in Colorado.
The key data is for 2011 just before legalisation and 2015 afterwards. Basically flat at 22% in 2011 and 21% in 2015. The rate for ever used in their life also flat being 39% and 38% respectively.
Still await with interest other data on drug use and abuse and associated harms before and after legalisation. But this data is useful in showing teenage use did not increase with legalisation.