Drugs fall into a difference category because they have a wider negative affect on society as a whole, and are therefore illegal for every citizen. I believe that drugs which only affect the user should be legal, from a purely libertarian standpoint, although I don’t think that drug exists.
And yes, I believe the issue is a binary one. If one is to be deemed by the state and adult, then that person should no longer be bound by laws that discriminate by age. I simply believe that should be the definition of an adult. If that age is deemed to be 21, then so be it. But I don’t believe the state should be allowed to tell some of its subjects that they are not yet responsible enough for a particular activity. I’m fine with the law deciding that children over a certain age are generally mature enough to do things like drive or get married, but I’m not okay with the statutes doing the same for adults. To take this argument to its logical conclusion, if there were a scientific, peer-reviewed, double-blind study showing that people aged 40-46 are 32% more likely to be involved in an automobile accident, would there be a case from restricting the right to drive for people in that age bracket? I would argue certainly not, and ages 18-21 should be no different, unless we legally consider them children.