The brain is one of the most important organs in our body. It controls all of the functions that go on inside our bodies, as well as those that allow us to learn and grow. In this section, we'll find out about the stages of brain development from birth to six years old. The first stage of brain development is called "pre-natal" or "fetal." This stage begins at conception and lasts up until birth. The second stage is called "early childhood." This starts at birth and lasts up through six years old. The third stage is called "middle childhood." It starts at six years old and lasts up through twelve years old. The fourth stage is called "adolescence." It starts at twelve years old and lasts up through eighteen years old, when a person is a full grown adult.