"Data on youth marijuana use was available from two sources. The Healthy Kids Colorado Survey (HKCS), with 47,146 high school and 6,704 middle school students responding in 2017, and the National Survey on Drug Use and Health (NSDUH), with about 512 respondents in 2015/16.
"HKCS results indicate no significant change in past 30‐day use of marijuana between 2013 (19.7%) and 2017 (19.4%). Also, in 2017, the use rates were not different from the national 30‐day use rates reported by the Youth Risk Behavior Survey.2 In 2017, 19.4% of Colorado high school students reported using marijuana in the past 30‐days compared to 19.8% of high school students nationally that reported this behavior.
"The 2017 HKCS found that marijuana use increases by grade level, with 11.0% of 9th graders, 17.7% of 10th graders, 23.7% of 11th graders, and 25.7% of 12th reporting use in the past 30‐days.
"The 2015/16 NSDUH, with many fewer respondents compared to HKCS, indicated a gradual increase in youth use from 2006/07 (9.1%) to 2013/14 (12.6%); however, the last two years showed decreased use, with 9.1% reporting use in 2015/16. The NSDUH showed that youth use of marijuana in Colorado (9.1%) was above the national average (6.8%)."
Impacts of Marijuana Legalization in Colorado: A Report Pursuant to Senate Bill 13-283. Colorado Department of Public Safety, Division of Criminal Justice, Office of Research and Statistics. October 2018, p. 5.