Actually in countries and states where it is legalized teen and young adult usage has gone down.

See: Colorado https://www.washingtonpost.com/news/wonk/wp/2016/12/21/one-of-the-greatest-fears-about-legalizing-marijuana-has-so-far-failed-to-happen/?noredirect=on&utm_term=.4d7151fd9e12

Raw Statistics on teen usage.

https://www.samhsa.gov/data/sites/default/files/NSDUHsaeShortTermCHG2016/NSDUHsaeShortTermCHG2016.htm