Several milestones in life are clearly defined by age. For example, at 18 years of age, you’re considered to be of legal age in the United States. But when it comes to brain development, scientists say there is no final maturity — our brains are constantly changing as we age.
In a study, published in the journal Neuron, neuroscientists agree that they do not know when adolescence ends and adulthood begins.
“There is no agreed-on benchmark that, when reached, would allow a neuroscientist to say ‘Aha! This brain is fully developed,’ ” said author of the study and Harvard University associate professor of psychology Leah Somerville.
But there is a good reason why the brain can’t reveal the exact moment when you’ve fully blossomed into adulthood. Sommerville says, it’s because different parts of your brain mature and develop at different times.
“Brain development occurs in waves, with different brain regions having their major developmental events at different times,” continues Sommerville. “So, the measurements that index brain maturation will give you different answers depending on what measure you’re focusing on and where in the brain you’re looking.”
Two types of tissue make up the brain — gray matter and white matter. During your first 10 years of life, gray matter expands and new synapses, or connections between nerves, are made. The more you learn and are exposed to new experiences as a child, the more gray matter grows.
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As puberty begins, your brain slows down on the gray matter surge and starts to boost up its white matter production. White matter is like the subway of the brain — it allows the various parts of the brain to share information better and faster. It also remains deep underneath the surface with many links and passages.
“The brain volume, the total volume, doesn’t really change, but we lose about one percent of gray matter starting around 13, and we gain about one percent of white matter at the same time, and that trade off keeps going,” says Dr. Jess Shatkin, a child and adolescent psychiatrist at the Child Study Center at NYU Langone Medical Center, who was not involved in the new paper.
The brain’s ability to interact with the environment, add new connections and grow new neurons over time is described as its plasticity. That’s what keeps change constant throughout life. So reviewing the volume of white matter is not a good way to classify maturity or immaturity, because it is something that is not defined by age. A study that offered proof of this found that some 8-year-old’s brains exhibited greater brain-connectivity maturation than some 25-year-olds.
“When considering whether an individual brain can diagnose someone as mature or immature, neuroscientists have deep concerns about trying to make those kinds of inferences. The very idea that we could come up with some number that would encompass all of the complexity involved in brain development is a challenge,” says Somerville. “While there are decades of evidence that adolescents behave differently from adults, the age of 18 doesn’t have any biological magic to it.”