From Dr. Jill Bolte Taylor's wonderful book My Stroke of Insight: Our brain cells do not multiply like other cells. Whether you are ten or eighty, you have the same cells (neurons) in your brain that you had when you were born! However, neural connections change with our life experiences. These differences in wiring contribute to our individual preferences and personalities. We have over one trillion cells in our nervous system, and over fifty trillion cells in our bodies.
The brain is divided into two hemispheres, left and right. They communicate with each other through an "information highway," the corpus callosum.
One of the features of the left hemisphere is the limbic system. That is where emotions are attached to the information traveling into the brain through the information highway. When we are newborns, these cells become wired together in response to sensory stimulation. These limbic systems do not mature. Therefore, we retain the ability to react to situations just like a two-year-old!
Then, how do we get the ability to respond in a more mature way? That's where the more complex (and more evolutionarily recent) cortical regions of the brain come in to play. These cells are neurons that we believe to be uniquely human. They enable us to choose a more mature response to situations.