Teen alcohol use is associated with these things plus much worse.

We are not legalizing for kids. Currently it is so available to kids that studies don't just ask if they can get cannabis, they ask how many hours it would take them to get it. A third of teens say it would take less than a few hours. Most have more than a couple of friends, so basically any teen can already get cannabis anytime they wanted. The federally funded Monitoring the Future Survey reports about 85% of high school seniors find marijuana "fairly easy to obtain". Their figure has changed little since 1975, never dropping below 81% in three decades of national surveys.

Regular teen access will not increase with legalization, and will likely decrease as the main supply channels are moved above ground where they can be easily monitored. At the very least we would take the cannabis they are getting from the criminal drug dealer of unknown character, with unknown potency, unknown purity (it could be laced, contaminated, etc), who never ID's, and put the supply in the hands of licensed, regulated, retailers who are not going to try and also sell hard drugs, or even have access to hard drugs.

The federal organization SAMHSA has shown that, despite greater acceptance and more lenient laws, the perceived availability of cannabis to youths aged 12 to 17 has dropped from 55% in 2002 to 48.6% in 2010. Multiple peer-reviewed studies have shown that medical marijuana laws have not led to increased teen usage [Choo et al. 2014; Lynne-Landsman et al. 2013; Harper et al. 2012; Anderson et al. 2012].

Teen usage dropped in Portugal since they decriminalized in 2001. The Netherlands have tolerated sales for years in 'coffee shops'. Both countries have lower usage rates, 3.3% and 5.4% respectively, than the U.S. which is 13.7%. Note that in the same time frame in which the war on drugs has been waging, tobacco use has dropped from about 45% to 18%, without criminalizing millions of tobacco users, whereas cannabis use went up.


Lenient cannabis policies are not associated with elevated adolescent use:

"the data provide no evidence that strict marijuana laws in the United States provide protective effects compared to the similarly restrictive but less vigorously enforced laws in place in Canada, and the regulated access approach in the Netherlands. " Simons-Morton et al. Cross-national comparison of adolescent drinking and cannabis use in the United States, Canada, and the Netherlands. Int J Drug Policy. 2010.