According to the focus on neuroscience box on the adolescent brain, the amount of gray matter in the brain peaks at about age for girls and age for boys.

11 year in girls and 12 years in boys

Explanation:

The volume of grey matter peaks at the adolescent age of child, however it starts growing since the childhood. It is considered that a girl’s adolescent age starts from 11 years while a boy’s adolescent age starts from 12 years. Thus, grey matter reaches at its peak when  girls reaches the age of 11 years and boy reaches the age of 12 years.

Grey matter starts declining as adulthood is reached in both male and female.