JULY 2004. 2. MCH BUREAU. EARLY CHILDHOOD. JULY 2004. 3. Research On Brain Development During Early Childhood. The human brain achieves approximately 85% of its adult size by age 2-1/2 years and 90% of total growth by the age of 3 years.Early childhood represents the period where young children attain developmental milestones that include emotional regulation and attachment, language development and motor skills. .