In high school, we learned that when a woman reaches 18 and men reaches 21, our brain is fully developed. But neuroscientists dismissed this notion because they knew that adults can still create new neurons and other brain cells and the network connections between those neurons are also in flux. Thus, there is no such thing as a fully developed brain. But in terms of volume size, the brain reached the maximum size at age ten.
Furthermore, in our teenage years, we trade more gray matter for white matter. Gray matter contains a bulk of neurons while white matter is the stuff connecting it all together. It is made of thin nerve fiber, axon surrounded by a fatty substance, myelin that electrically insulates axons and speeds along with brain signals. The reason why teenager's brains trading for white matter is to enhance the speed of information transmission of neurons. Also, the brain is going through synaptic pruning, where an intense spring cleaning and eliminating unused connection happened to help the brain thinks more efficiently, and strengthen the connections between the different regions.
Additionally, by the time you're 20 the visual center at the back of the brain, the occipital lobe is wrapped up but when it comes to the frontal lobe, responsible for decision making and problem-solving, it takes another decade to do the same. Thus, some researchers believed that the definition of adulthood should be a person capable of controlling their emotional responses.