By what age does the brain reach nearly 90% of its adult weight?

The brain reaches nearly 90% of its adult weight by the age of three years. During the early years of development, significant growth and brain maturation occur, with the most rapid expansion happening in the first few years of life. This is a critical period where the foundational neural connections are formed, affecting cognitive development, language skills, and social-emotional growth.

By the age of three, children have made considerable strides in their cognitive abilities. This developmental milestone reflects not just physical growth but also the establishment of synaptic connections that are crucial for learning and development. The brain's rapid growth during this period highlights the importance of rich environmental interactions, stimulating experiences, and early education to support optimal learning and development in young children.

In contrast, the brain's growth trajectory before this point, such as at one or two years, shows substantial growth but does not yet reach the 90% mark, while at four years, the brain is indeed approaching its adult size, but three years is the defining age where this milestone is achieved.