Total US
According to the National Survey on Drug Use and Health (NSDUH), in 2008-2009, an estimated 13.37% of young people in the US aged 12 through 17 had used marijuana in the past year, falling to 12.29% in 2015-2016. The NSDUH also estimates that, in 2008-2009, an estimated 7.03% of young people in the US aged 12 through 17 had used marijuana in the past month, dropping to 6.75% in 2015-2016.

Colorado State
According to the NSDUH, in 2008-2009, an estimated 18.55% of young people in Colorado aged 12 through 17 had used marijuana in the past year, falling to 16.21% in 2015-2016. The NSDUH also estimates that, in 2008-2009, an estimated 10.17% of young people in Colorado aged 12 through 17 had used marijuana in the past month, dropping to 9.08% in 2015-2016.

Table: Prevalence of marijuana use in Colorado before and after passage and implementation of Amendment 64

Source

Substance Abuse and Mental Health Services Administration. (2017). National Survey on Drug Use and Health: Comparison of 2008-2009 and 2015-2016 Population Percentages (50 States and the District of Columbia). Rockville, MD: Center for Behavioral Health Statistics and Quality, Substance Abuse and Mental Health Services Administration.
Substance Abuse and Mental Health Services Administration. (2017). National Survey on Drug Use and Health: Comparison of 2014-2015 and 2015-2016 Population Percentages (50 States and the District of Columbia). Rockville, MD: Center for Behavioral Health Statistics and Quality, Substance Abuse and Mental Health Services Administration.