When do we truly become adults? Is it when you turn 18 and leave home, or the moment you realize that you’re responsible for your own appointments? Or perhaps, like my father, you still feel young, despite the mirror revealing your age?

Legally, adulthood often starts at 18 or 21 in various countries, giving you the power to vote, marry, and make medical decisions. However, the journey of brain development is far more intricate. The brain transitions from a juvenile to an adult state gradually, without a definitive moment of transformation. Some brain networks mature in early adolescence, while others develop well into your 20s and beyond.

When can we begin to hold ourselves accountable for our actions as adults? The timeline is more extended than you might estimate.

Until recently, neuroscientists believed that brain maturity was reached around age 25, though there was no solid biological benchmark for this claim. This notion gained traction in the early 21st century from studies that analyzed brain development up to age 20. Since the data was limited, the age 25 estimate offered a broad buffer for individual variations.

Recent research aims to identify precise ages by examining behavior linked to specific brain development stages. For instance, gray matter—dense tissue rich in neurons and synapses—typically thins during the teenage years before stabilizing in the 20s. According to research led by Christian Tamnes from the University of Oslo, gray matter thickness tends to decrease through adolescence and plateau in adulthood.

This brain thinning isn’t alarming; it indicates a transition from a tangled web of connections in childhood to a more streamlined network in adulthood, akin to upgrading from a winding back road to a well-designed highway.

Nevertheless, gray matter doesn’t mature uniformly, revealing that maturity isn’t a fixed point. Factors like socioeconomic status, culture, and social circles play significant roles in brain development. For example, stressful experiences during adolescence may promote gray matter reduction, particularly in lower-income households.

It might be more pertinent to ask: when does the brain begin to exhibit adult behaviors? We can define adulthood through executive function—the capacity for rational decision-making, self-control, and future planning. “Executive function serves as a valuable indicator of brain maturity,” notes Brenden Tervo-Clemens, a researcher focused on normative brain growth at the University of Minnesota.

To explore this, Tervo-Clemens and his team analyzed data spanning four significant datasets involving over 10,000 individuals aged 8 to 35. Their findings reveal that executive function evolves rapidly between ages 10 and 15, experiences smaller but noteworthy changes from 15 to 17, and stabilizes around 18 to 20. Thus, according to this scale, the adult brain reaches full maturity by age 20.

Another facet of adult development is the social cognition intricacies within brain networks, enabling interpersonal interactions. A study by Philip Jackson and colleagues at Université Laval explored brain maturity from ages 12 to 30, revealing varying rates of social function maturation. Skills such as understanding others’ intentions tend to solidify during early adolescence, while the capacity for empathy continues developing post-18.

However, focusing on a singular ability for adult definition can oversimplify the complex nature of the brain. “The brain operates as an intricate system with multifaceted interactions,” observes Tervo-Clemens. “Attempting to find a single measure of brain maturity will always be reductive.”

To gain a comprehensive overview, Alexa Mousley, a developmental neuroscientist at the University of Cambridge, recently analyzed brain scans from infancy to 90 years. Their study, published last year, explored white matter pathways—vital connections facilitating communication between various brain regions.

They identified four critical transformation periods during development, occurring around ages 9, 32, 66, and 83. The timeline from ages 9 to 32 seems especially relevant for defining adulthood, as the brain transitions from fragmented communication during childhood to a more integrated network in adulthood, peaking in global efficiency at age 29.

A separate study from May further corroborates these findings, indicating that while certain white matter areas achieve peak maturity in our 20s and 30s, others continue developing into our 40s. This reinforces the understanding that brain refinement extends well beyond the legal definition of adulthood.

Despite the discrepancies in timelines, these studies indicate that full brain maturity does not occur at age 18, with tangible effects in everyday life. According to Katia Rubia, a Cognitive Neuroscience professor at King’s College London, the limbic system, responsible for emotional processing and reward generation, often matures during adolescence. In contrast, frontal lobe networks governing emotion regulation, impulse control, and foresight may continue developing much later, resulting in an imbalance where adolescents often engage in impulsive actions.

Rubia urges policymakers to consider these brain development insights, suggesting that legal driving ages should be revised. She notes that many accidents involve adolescents whose frontal lobes aren’t yet fully developed, leading to riskier driving behavior.

Some scientists propose developing brain growth charts akin to regular height and weight measurements, facilitating comparison against normative data in contexts like criminal sentencing. However, this remains a challenge. The 2020 report for the Scottish Sentencing Council indicated that logistic concerns make widespread implementation impractical, but as research grows, this may become feasible.

Fundamentally, our legal, medical, and social frameworks require a clearer definition of adulthood, one that the nuances of neurology currently can’t provide. Brain development is uneven and personal, shaped by genetics, culture, and experiences. Certain networks mature faster than others, and some brain functions, like white matter pathways, may not reach full maturation until the 40s, while others decline earlier. Adulthood, therefore, isn’t a fixed endpoint but a continuous journey of growth and change.

Emotionally, research indicates that individuals often feel they reach a sense of maturity around age 29. Thus, while legally we transition to adulthood at 18, neuroscience suggests this development continues well into our 20s and even into the 40s, with personal growth unfolding at its own pace. My father, now 81, still waits for his moment of maturity.