I believe Cortex is the last structure of the brain to complete its growth process. Surrounding the brain is the Cerebral cortex which resembles a half shelled walnut, it is the largest, most complex brain structure which accounts for 85% of the brain's weight, containing the greatest number of neurons and synapses, and responsible for the unique intelligence. It is the last brain structure to stop growing, it is sensitive to environmental influences for a much longer period than any other part of the brain.