Even if we grant the notion of childhood as valid (one which I agree with, but am open to other models - the idea that one turns 18 and is suddenly a different person has weird impacts), we are seeing a systemic erosion of the notion. One which makes the GP's question valid.

I mean that the very idea of 18 == adulthood is going away. For instance, more and more people over the age of 13 but under the age of 18 are being prosecuted as adults in criminal cases. Zero tolerance policies for kids of all ages are assuming that they are capable of making choices with life altering consequences, but only in negative ways. So for some actions - adult understanding of the world is assumed much younger than the age of 18. On the other hand, we are seeing the infantilization of people older than 18 - drinking age is 21, car rental 25, college policies are becoming more and more like those of high-school, college campus law enforcement is becoming less and less tolerant of "young people behavior" (e.g. parties and festivals). It is a weird set of standards that sends a strange message. Those that conform well are placed in a class that can fully enjoy the rewards of membership eventually, and those that don't or make mistakes are placed in a different class, one with many restrictions.

Basically I'm saying that while the notions of adulthood and childhood aren't bad, the implementation brings them into question.