As more is understood about brain development after adolescence, it seems that the characteristics of postformal thought are evident as the brain and, in particular, as the matures

The best answer to the question, regarding the development of the brain in teenagers, and especially regarding the appearance of post-formal thought processes, is: the maturation of amygdala and later on the pre-frontal cortex, in later adolescence and early adulthood.

Explanation:

Although the question is pretty tricky, as there is still research being done on what, how and when portions of the adolescent brain start to change so that thought processes also mature, it is now known that the staring portion that develops and matures during this period of human life is the amygdala. Given that it plays a role in emotions, aggression, and typical reactions to stimuli in adolescents, it explains why their decisions tend to lean towards emotionality, rather than pure logic. During the early stages of adolescence, until late adolescence, it is the amygdala that mediates the thought processes, and behavior, of adolescents, while the pre-frontal cortex, the region for higher thinking, continues developing mostly during the late adolescent years and early adulthood, around the 20 to 25-year mark.