Final answer:
The brain increases in size significantly during infancy and childhood, and by the age of six, it has already attained about 90 percent of its adult size. The changes in brain size and complexity during this period contribute to major cognitive changes.
Step-by-step explanation:
The brain increases in size significantly during infancy. By the age of six, the brain has already attained about 90 percent of its adult size. During early childhood, the frontal lobes grow rapidly, and the brain growth spurts experienced in childhood tend to follow Piaget's sequence of cognitive development. These changes in brain size and complexity during infancy and childhood contribute to major cognitive changes.