Synaptic pruning and selective elimination during adolescence
An answer to this question could potentially be found in "Understanding Adolescent Brain Development and Its Implications for the Clinician"  


  Thanks to research by Jay
  Giedd and colleagues at the National Institute of Mental Health, it has become
  clear that, during the adolescent years, the organization and functioning of the
  brain go through complex changes. Importantly, these changes seem to be unique
  to the adolescent years and not simply the trailing remnants of childhood brain
  development.


Changes in frontal lobes during adolescence 


  Frontal lobe gray-matter volumes,
  which represent dense concentrations of neurons and their parts, increase
  throughout childhood and do not reach their peak until approximately the age of
  11 (girls) or 12 (boys), at which point they decline throughout the second decade
  of life and into young adulthood.


So why might frontal lobe gray-matter volumes go up during childhood and down
during adolescence?


  Recent data suggested that, during childhood, neurons in the
  frontal lobes are allowed to overgrow and form far too many points of communication,
  or synapses, with other neurons. As a result, gray-matter volumes
  increase. As childhood draws to a close and adolescence begins, the brain
  switches from overproduction mode to selection mode. Early in the second
  decade of life, the brain stops overproducing synapses in the frontal lobes and
  puts the synapses that exist on the chopping block. Hundreds of billions of points
  of communication will be sacrificed through the teenage years. Only those that
  form meaningful, useful points of contact will be kept. Guided by a teenager’s
  experiences, the frontal lobes are shaped and molded into a configuration that
  will carry the individual, for better or worse, through the adult years. As this
  pruning process unfolds, gray-matter volumes decrease.


Changes in other parts of the cortex (specifically temporal lobes)


  The temporal lobes, which are critical for memory formation as well as processing
  auditory information and seeing detailed patterns and shapes, do not reach
  their maximum levels of gray matter until the age of 16 to 17, at which point they
  plateau. The temporal lobes contain the hippocampus, a structure that is central
  to creating an autobiographical record of what one does and what one learns.


Sources: 


White, Aaron M. "Understanding Adolescent Brain Development and Its Implications for the Clinician." Division of Medical Psychology, Department of Psychiatry, Duke University Medical Center, Box 3374, Durham, NC 27710, USA, 2009. Web. 7 Feb. 2016.
Steinberg, Laurence. "A Social Neuroscience Perspective on Adolescent Risk-Taking." Developmental Review : DR. U.S. National Library of Medicine, 28 Mar. 2008. Web. 07 Feb. 2016.