Early brain development
A baby's brain develops through experiences, gained during certain periods of its development. For example, the circuits connecting the eyes to the brain mostly wire up as we look around during the first few weeks after birth. Different circuits wire up at different times during infancy - we learn to smile at about 6 weeks, crawl at about 9 months and walk at 12 months. However, the brain is quite flexible - if any of these periods are missed, most circuits are still able to wire up until about 10 years of age.