Story highlights Even neuroscientists don't know when adulthood begins, according to a new paper

Your brain is still maturing in your 20s and even early 30s, experts say

(CNN) You're legally an adult when you turn 18, in most of the United States. However, from a scientific perspective, adulthood is an unsolved mystery.

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 on Wednesday.

"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.

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