By age five or six, a child's brain is 90 percent the size of an adult's, and for a long time scientists thought that the organ's significant structural growth ended by around 12 years old. Recent research, however, shows that an adolescent's brain makes dynamic changes around that age as well as during all of the teen years. As Leslie Sabbagh explains in our cover story, “The Teen Brain, Hard at Work,” beginning on page 20, areas involved in planning and decision making experience a spurt of growth at 11 or 12 years and then undergo pruning and reorganization through the early 20s.