The brain is 'plastic' - synaptic connections form and are pruned.
During infancy, the brain experiences a rapid growth in synaptic connections, peaking at about 15,000 at age 2-3 (Gopnick).
As we age, rarely used connections are deleted and the frequently used are strengthened - synaptic pruning.
It was once thought that these changes were limited to childhood. But recent research suggests neural connections can change or be formed at any time, due to learning and experience.