Teen marijuana use does not appear to be increasing in states that have legalized marijuana.
A federal study in all 50 states showed that overall, teen use has remained flat and has decreased in some states. The data, however, has been misinterpreted or misused by anti-legalization groups to try and make a case against legalization.
Colorado had the 4th highest teen "past-month" use in the nation before legalization in 2012. The most recent survey, conducted in 2015, showed that Colorado was now 1st in this category. It turns out though, that the change was because other states had decreasing use while Colorado use was unchanged.
Keep in mind that marijuana is still illegal for teens without parental consent and a medical marijuana recommendation. Despite this, teens report that marijuana was readily available both pre- and post- legalization.