One reason the early teen years are cited as a line of demarcation, it that it is at this point where NT development significantly diverts to adulthood from childhood. The sullen withdrawal during the early teens years has been likened to someone who suddenly awakes with telepathy. It is overwhelming to be considering deeper meaning that one has been previously oblivious to. For example, an eight year old upon hearing someone declare, "You stink", would not be to disconcerted. However, a teen girl who hears someone make the same comment to her might be horrified to consider the possible implications (that she does smell, that people hate her, or that someone she trusts has betrayed her).
The transition into the perception and processing of deeper and abstract thoughts can take years to assimilate just as if someone were to become telepathic would take years to sift through the unwanted to find the essential.
It is not uncommon fro Asperger people to resist the conforming influences that so much shape the development of NT people. The Asperger child often has a strong idea of what he likes and is reluctant to exchange it for something without being convinced of superior value.
In a similar way, it is difficult to put forth a convincing argument for why Peter Pan should grow up. NTs are often on a path of maturity that is created by peers, the expectations of others, and a lemming like patterning of their environment.
Since so much of Asperger life is self-directed, these conforming influences often have little effect resulting in developmental delays.