Despite my libertarian conviction that consenting adults must have the right to do anything they damn well please so long as they cause no direct harm to others, I recognize "adulthood" as a condition that children don't have by definition. I don't think they should take drugs of any sort until their bodies, minds, emotions and hormones have attained maturity. Which for most people is in the very late teens and for many is a couple of decades later, but that's a topic for another thread.
That said, as a libertarian I am also thoroughly opposed to a government that can only enforce its laws by coaxing citizens into informing on one another. That's fascism. Bitch'n'snitch programs make everyone distrust everyone else, and trust is the cornerstone of civilization. If large numbers of people don't want to obey your laws, then your laws are shit.
The reason these kids are drinking is that their parents just don't give a flying fuck. You can't throw all of their parents in jail because there isn't enough foster care to take them in. I think this is a problem we're going to have to live with because the alternatives are worse.
Although we could mitigate it by ending the War On Drugs. I know from reading the statistics that teenage drinking has increased tremendously since marijuana has become more expensive and since drug testing has become prevalent--which persecutes users of marijuana more than any other drug since the metabolites stay in your system for weeks. I also know from personal observation that hordes of adults also started drinking more alcohol and smoking less weed, for the very same reasons. Finally, I know from (very long ago) personal experience that marijuana is far less dangerous for adults than alcohol, and I can only suppose that it's the same for teenagers.
All it takes is one good reason and there is one. Drunken driving is a major cause of death among teenagers, and stoned drivers are immeasurably safer drivers than drunk drivers.