Most people are considered to be legally grown-up by the time they reach 18 years of age but neuroscientists said that the human brain may not have fully reached maturity at this age yet.

Human Brain Undergoes Constant Change

In a new paper, which was published in the journal Neuron on Wednesday, Dec. 21, Leah Somerville, from Harvard University, said that the human brain constantly changes and it is not clear when adolescence ends and adulthood starts in the brain.

Somerville, who wrote the commentary amid concerns over the importance of the concept of maturity particularly among policy makers and for the criminal justice system, said that it is safe to say that the brain continues to develop actively past the age of 18 and that different parts of the brain reach maturity at different times.

"Brain maturation is a multi-layered process that does not map on to a single developmental timeline," Somerville wrote. "On the gross structural level, the developing brain exhibits reductions in cortical gray matter and increases in the volume and anisotropy of white matter from childhood to adulthood."

Gray Matter Pruning And Specialization Of Different Parts Of The Brain

The brain has two types of tissue, the gray matter and white matter. Up until a person reaches 10 years old, the gray matter grows and rapidly forms new synapses that form connection among nerves. The gray matter grows as a child is exposed to new information and experiences.

The brain, however, starts to prune back some of the gray matter as the body prepares for puberty, a phenomenon known as gray matter pruning. Pruning, which was also observed in new mothers, plays a role in the specialization of the different parts of the brain. When the brain starts pruning back some of the gray matter, it also boosts the production of white matter.

Jess Shatkin, from the NYU Langone Medical Center, explained that pruning does not change the total brain volume. At age 13, people lose about 1 percent of the gray matter and gain about 1 percent of white matter.

When Does The Brain Reach Adulthood?

Most neuroscientists agree that there is no such thing as a magic age at which the brain reaches adulthood. Shatkin said that the brain still goes through big changes until the early 20s and even until the early 30s.

"We don't really know when this process of development ends. ... Twenty-five, 26, 28, 30, 32? We don't know," he said.