"Act your age" says one group, while "you are only as young as you feel" assures another. Those are two conflicting messages about aging and maturity. How can anyone know when maturity is "just right"?
Neuroscientists actually don't know any more than people who create clichés about when your brain is a legal adult, but at least they get to take pretty pictures and speculate about what they mean. The legal system gets no such luxury, they have to draw a line between adolescence and maturity, and defenders and prosecutors both claim to shroud themselves in science when they do it.
But it's not that simple. Different parts of the brain mature at different rates, from around age 10 to the early 20s. Defining when a brain 'reaches maturity' is much trickier than it may seem.
In an op-ed in Neuron, psychologist Leah Somerville notes that no one can even agree on what 'mature' means, which helps explain why DSM 5 is such a mess and can barely be used as a glossary for psychiatry. Research has shown that while there are clear structural differences between an adolescent and adult brain (these include a reduction in gray matter and increases in white matter), brain maturation does not map on to a single developmental timeline, and it differs by individual when different parts of the brain develop. In one large study, several regions of the brain had not yet plateaued even by the age of 30.
The plasticity of the brain -- its ability to interact with the environment, add new connections and grow new neurons over time--also makes it so that change is constant throughout life. A simple look at the volume of white matter or connection patterns between brain cells would not be an effective way to identify a static baseline that defines maturity or immaturity. For example, one study found that some 8-year-old brains exhibited greater brain-connectivity maturation measures than some 25-year-old brains.
"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," Somerville says. "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. While there are decades of evidence that adolescents behave differently from adults, the age of 18 doesn't have any biological magic to it."