Even neuroscientists don't know when adolescence ends and adulthood begins in the brain -- after all, our brains are constantly changing, according to a new opinion paper published in the journal Neuron
"There is no agreed-on benchmark that, when reached, would allow a neuroscientist to say 'Aha! This brain is fully developed,' " said Leah Somerville, an associate professor of psychology at Harvard University, who wrote the paper.
"However, it is safe to say that by almost any metric, the brain is continuing to develop actively well past the age of 18," she said. "The reason I think it's important to discuss this issue is because policies impacting youth have begun to pay more and more attention to the concept of neurodevelopmental maturity, so neuroscientists have begun to get engaged in these complicated discussions," such as debates about when to charge a child with an adult crime or when to permit aging out of the foster care system
So, why can't the brain reveal whether you're an adult or not? Different parts of your brain mature at different times, Somerville said.
"Brain development occurs in waves, with different brain regions having their major developmental events at different times," she said. "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."
In other words, since there is no definitive way to measure brain maturity, there is no definitive age in which your brain signals that you are an adult.
Your brain consists of two types of tissue, gray matter and white matter
. In your first decade of life, the gray matter grows and expands rapidly as many new synapses, or connections between nerves, are being made. The gray matter grows as you learn and are exposed to new experiences as a child.
Then, as your body prepares for puberty, your brain starts to prune back some of that gray matter and amp up its production of white matter, which allows different parts of the brain to share information better and faster, said 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 volume, the total volume, doesn't really change, but we lose about 1% of gray matter starting around 13 and we gain about 1% of white matter at the same time, and that trade off keeps going," Shatkin said.
"There are big changes (in the brain) until the early 20s, and there may be pretty significant changes still until the early 30s," he said. "We're still learning. Whereas, once upon a time, we imagined adolescence to end at 18, now we don't really know when this process of development ends. ... Twenty-five, 26, 28, 30, 32? We don't know."
Shatkin added that this maturation process of the brain tends to appear in female brains earlier than male brains.
"There's been some neurological data showing that girls' brains may begin to prune earlier," he said. "This certainly goes along with what we typically observe clinically as well."
Overall, most neuroscientists agree that "there's no magic age at which the brain reaches adulthood," said Jeffrey Jensen Arnett, a research professor of psychology at Clark University in Massachusetts.
"I've been waiting to see a paper like this, and have thought of writing one myself," he said. "I've done a lot of research on how people define adulthood, and people vary, but in general it's not an age, it's defined by qualities such as accepting responsibility for yourself and making independent decisions."