Final answer:
The brain and head grow the most rapidly in infancy, with about 90 percent of adult brain size being reached by the age of six. During adolescence, the brain becomes more complex rather than significantly larger. The frontal lobes fully develop by age 25.
Step-by-step explanation:
The brain and head grow more rapidly than other parts of the body in infancy. This period of rapid growth continues until about 90 percent of the adult brain size is achieved by the age of six. Although the frontal lobes become fully developed by the age of 25, the most substantial growth in size occurs earlier.
During adolescence, the brain does not significantly increase in size but undergoes considerable developments in complexity, including synaptic pruning and myelination. Adolescents experience cognitive development with their basic thinking abilities becoming comparable to adults by age 15, with further maturation continuing into their early 20s.
It's important to note that after early childhood, growth slows down and the development of the brain in adulthood pertains more to functional and structural refinement rather than size. As such, infancy is the correct answer to the question concerning the rapid growth of the brain and head.