At what age is the brain fully mature in size?

The brain reaches its full size by age three. During the early years of life, particularly from birth to around age three, the brain undergoes rapid growth and development. By this age, significant neural connections are established, and the overall size of the brain approaches its adult size.

While the brain is functional at birth, it is not fully developed in terms of size and complexity. By the end of the first year, the brain grows remarkably, but it continues to develop in both structure and function until about the age of three, when most of its major growth in size occurs. After this point, while the brain continues to develop connections and pathways, the increase in size is minimal compared to the rapid growth seen in the first three years of life. Full maturity in terms of complex executive functions and cognitive abilities typically occurs later, reaching completion by adolescence. Hence, understanding brain maturity must consider both size and functional maturity over time.