Final answer:
The brain is about 25% of its adult size at birth and grows to about 90% by the age of six, with neurogenesis, followed by synaptic pruning and myelination through adolescence.
Step-by-step explanation:
At birth, the brain is about 25% of its adult size. However, by the age of six, the brain has rapidly grown to reach approximately 90% of its adult size. This increase in brain size correlates with significant cognitive and motor skill advancements in early childhood, during which a process of neurogenesis occurs, followed later by synaptic pruning and myelination through adolescence, enabling increased brain efficiency and function.