At the same time that young adults are experiencing new levels of sophistication in thinking and emotional regulation, their brains are undergoing changes in precisely the areas associated with these functions. While it is not possible to determine cause-and-effect, brain and behavior are changing in parallel.
Pre-frontal cortex: The most widely studied changes in young adulthood are in the pre-frontal cortex, the area behind the forehead associated with planning, problem-solving, and related tasks. At least two things affect the efficiency in its functioning:
myelination: the nerve fibers are more extensively covered with myelin, a substance that insulates them so that signals can be transmitted more efficiently, and
synaptic pruning: the "briar patch" of connections resulting from nerve growth are pruned back, allowing the remaining ones to transmit signals more efficiently.
Connections among regions: At the same time, the pre-frontal cortex communicates more fully and effectively with other parts of the brain, including those that are particularly associated with emotion and impulses, so that all areas of the brain can be better involved in planning and problem-solving.
http://hrweb.mit.edu/worklife/youngadult/brain.html#text