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When did you truly become an adult? Maybe it was the day you turned 18, left home or realised – with sudden horror – that no one else was going to book your dental appointment. Perhaps, like my dad, you still don’t consider yourself a grown-up, despite the wrinkled face in the mirror suggesting otherwise.

Legally, adulthood arrives on a schedule, usually at age 18 or 21 in most countries. Once you hit that magic number, you can make your own medical decisions, vote or marry. Yet neurologically, it isn’t that straightforward – there isn’t an exact moment when the brain flips from a juvenile to adult state. The reality is far messier, with some networks attaining adult-like status in our early teens, while others continue maturing throughout our 20s and beyond.

So, when can we stop blaming our developing brain for slip-ups and immaturity and face up to adulthood? The answer is later than you might think.

Until about a decade ago, neuroscientists tended to agree that the brain fully develops by age 25 – but this figure was never pinned to a particular biological threshold. Instead, the idea seems to have emerged from several influential studies around the turn of the 21st century, which tracked brain development only to around age 20. Because the data stopped there, 25 became a sensible estimate that accounted for potential variations in development. The number was cemented in popular wisdom.

Since then, more researchers have attempted to zero in on a more accurate number by looking at development across specific brain structures, as well as behaviours associated with maturity. One place to start is grey matter – the tissue packed with neurons and synapses, where most processing occurs. In 2017, , a neuroscientist at the University of Oslo in Norway and his colleagues published that showed grey matter thickness generally declines during our teens before levelling off in our twenties.

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This isn’t cause for alarm. In childhood, the brain has excessive neuronal connections, making it a tangle of possibilities. It prunes them away throughout adolescence, so that networks become more efficient. Think of it like replacing a jumble of back roads with streamlined highways.

Yet grey matter , meaning there still isn’t one turning point for maturity. Not everyone follows the same trajectory, either – your socioeconomic status, culture and peers all shape brain development, too. For instance, experiencing hardship or stress in adolescence can accelerate grey matter thinning, as can growing up in a low-income household.

Perhaps, then, the real question isn’t when the brain starts looking like an adult’s, but when it behaves like one. Maybe we should define adulthood by executive function – the capacity to make rational decisions, suppress inappropriate or risky behaviour and plan for the future. “Executive function is a good way, perhaps among the best, to think about so-called brain age and brain-based maturity,” says , who researches normative brain development at the University of Minnesota.

To explore this idea, he and his colleagues analysed , pooling data from more than 10,000 people aged between 8 and 35. They showed that executive function undergoes a rapid burst of development around 10 to 15 years of age, followed by smaller but significant changes between 15 and 17, before finally stabilising between the ages of 18 and 20. By this measure, adulthood arrives at around age 20.

Another candidate for determining adulthood is the brain networks’ underlying social cognition – the mental processes that enable us to interact with others. Here, too, there isn’t a single timetable. In 2017, Philip Jackson at Laval University in Canada and his colleagues and found that different aspects of social function mature at different ages. Some are cemented into neural architecture in early adolescence, like the ability to infer the intentions and desires of others, known as social knowledge and mentalising skills. Meanwhile, adolescents’ ability to show empathy continued developing past 18.

However, it may be misguided to fixate on single abilities. “The brain is a complex system involving ultra-high-dimensional interactions,” says Tervo-Clemmens. “Therefore, the search for any single measure to determine brain age is necessarily an oversimplification.”

To get a broader view, , a developmental neuroscientist at the University of Cambridge, and her colleagues recently mapped brain development across the entire human lifespan, analysing brain scans from newborns all the way up to 90-year-olds. The study, , focused on white matter tracts, the projections that connect brain cells to one another, allowing different regions to communicate.

They found these tracts undergo four major shifts in the march towards maturity, around ages 9, 32, 66 and 83. The period between ages 9 and 32 seems especially pertinent for defining adulthood. During childhood and adolescence, the brain appears relatively segmented, with clusters of regions talking primarily among themselves. As brain development continues, these boundaries become less defined, and communication across the brain grows increasingly integrated. Known as global efficiency, this measure peaks around age 29.

A separate reached a similar conclusion. It analysed more than 35,000 brain scans, showing that some white matter regions reach their developmental peak in the 20s and 30s, while others don’t until the 40s. That suggests that, at least by this measure, the brain continues to refine itself far beyond the legal age of adulthood.

Despite a lack of consensus, what all of these studies make clear is that the brain isn’t fully developed at age 18 – which has real-world consequences, says , a professor of cognitive neuroscience at King’s College London.

For instance, limbic regions, which are involved in generating emotions and processing reward, mature in adolescence. Yet the frontal lobe networks that control those emotions and are involved in planning, impulse control and foresight may develop far later. The result is a familiar imbalance: “In adolescence… there are far more impulsive behaviours, such as shoplifting, teenage pregnancy, car accidents [and] substance abuse,” says Rubia. “This is because the frontal lobe is not yet fully developed, and hence young people do not think or care that much about the future consequences of their acts or plan ahead because these functions are mediated by the frontal lobe.”

Rubia argues that policy-makers should take this into consideration. “In my view, driving licences should be given later,” she says, noting that most accidents happen in younger people because an underdeveloped frontal lobe results in riskier and less foresighted driving.

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Other academics have suggested creating brain development charts that track typical maturity, much like those that doctors routinely use for height and weight. That way, we could compare individuals to the norm, which could be useful for matters such as criminal sentencing. But that currently isn’t feasible. “The widespread use of imaging is both impractical and unlikely to be helpful given the variability between individuals,” wrote the authors of a 2020 for the Scottish Sentencing Council, an independent advisory group that helps prepare criminal-sentencing guidelines. But as the number of studies mapping brain development grows, it may be possible in the future, they said.

What’s clear is that while legal, medical and social systems need an exact definition of adulthood, neurology can’t provide one. Maturity in the brain progresses unevenly and doesn’t look the same for everyone – genetics, culture and experience all shape it. Some systems mature surprisingly early, whereas others do so later than we might like. A handful, like the white matter tracts facilitating brain communication, may even develop well into your 40s, when other networks start to decline. On that basis, adulthood isn’t an endpoint but a gradual handover.

And emotionally? That is another story. Global surveys suggest people feel “grown up” at around age 29. So, while the law often bestows adulthood at 18 and neuroscience suggests it sits somewhere between 20 and the mid-40s, your sense of maturity seems to arrive at its own pace. My dad is 81. And he is still waiting.