I fully support dangerous drivers being removed from the road when it is proven that they were doing something dangerous while driving. That's how our legal system (was supposed to)work(s). Guilt is proven by the person who has been violated when the trnasgression is made. Inocence is not proven in advance to excercise a right.
You start with the presumption of guilt, i.e. that just becuase they are of a certain age, they may be using "illegal" intoxicants and must prove their innocence.
Again, I agree with removing drivers shown to be dangerous. But where libertarians always miss is in situations where we know that certain freedoms (like the freedom to do drugs and drive) will cause a certain significant percentage of the populace to do so - thus endangering our families. Telling a kid he has to be drug free to get his license does NOT imply guilt. It simply acknowledges that some kids DO do drugs and apply for licenses and that the citizenry thinks that's a bad idea. A kid who doesn't do drugs is in no way told he's guilty of anything. A kid who does will have to make a choice.