I believe it is at the age of 6. Virtually all human behavior is governed by the brain. Experience that produces a change in behavior produces a change in the brain. Increase in Brain size. 25% of adult weight by birth, 50% of adult weight by 6 months, 75% of adult weight by 2 years, 90% of adult weight by 4-6 years, 95 % of adult weight by 6-7 years.