Marijuana opponents go to the 'what about the youth' argument early and often. They always have, and they probably always will. Opponents will try hard to create a hypothetical future in which society has gone in the dumps because a generation of teenagers consumed cannabis. It's the same argument that opponents have been making for decades, yet the teenager stoner zombie apocalypse is somehow avoided year after year.
Marijuana regulations work to keep marijuana out of the hands of youth. As activists always point out, a licensed and regulated outlet will ask for ID whereas the black market won't. Licensed and regulated outlets operate in the open, whereas black market deals are done in the shadows. Disputes in a legalized and regulated market get handled in court and mediation, whereas disputes in the black market often get handled in a much less civil fashion.
Marijuana legalization is a good thing, as proven by a recent study that made a lot of headlines today. Per Science Daily:
An abstract of the study, "Adolescents' Ease of Access to Marijuana Before and After Legalization of Marijuana in Washington State," will be presented at the Pediatric Academic Societies 2016 Meeting in Baltimore on Sunday, May 1. Researchers compared 2010 and 2014 data from the Washington State Healthy Youth Survey. Each year's survey included questions about ease of access to marijuana, alcohol, cigarettes and other illicit drugs.
There was virtually no change in the proportion of teens who reported it was "easy" to access marijuana in 2010 (55 percent), compared to 2014 (54 percent) after the new law was enacted, according to the study.
These numbers will of course be tracked forever in all legal states. There may be years where it goes up a percent, but I think for the most part the number will continue to go down as regulation continues to work. There's still a big black market in Washington because of the current rules, but I'm hopeful that activists there will keep putting pressure on elected officials to improve the state's marijuana laws.