* Joan of Arc was in her teens when she led a platoon into battle.
* Galileo was 19 when he made some of his most dramatic discoveries about the density of matter.
* Newton was 23 when he had his Eureka moment about gravity.
* Mozart was touring Europe and performing before royalty by the age of six.
These were prodigies, certainly. But given how low life expectancy was until the modern era, researchers now believe that a large share of humanity’s most dramatic discoveries—harnessing fire, the wheel, smelting, and plenty more — were likely made by people who were quite young.
Necessity drove invention, and necessity meant there were few rules about the burdens young people could carry.
The rules that society has evolved in recent centuries are, of course, vital. But new studies indicate that, amid current societal structures, the threshold of adulthood may be moving further up. We are now at the point where humans may not be maturing into full adulthood until the age of… 32.
A study by neuroscientists at Cambridge University suggests that most individuals reach true adulthood in their early 30s. That is when the brain’s process of pruning and rewiring its neural networks reaches peak structural efficiency. This peak holds for a few years, and then a gradual decline begins, often in the mid-30s.
“Our work highlights that brain development is slow and non-linear, and that neural networks reach maximum efficiency by an average age of 32,” says Alexa Mousley, a neuroscientist and postdoctoral research associate, and lead author of the study, which was published in Nature Communications in November.
In their study, the researchers examined the MRI scans of 3,802 individuals ranging from infants to 90-year-olds. Based on the rewiring achieved by the brain, they have revised the phases of the human lifespan as…
* Childhood: Birth to 9 years
* Adolescence: Ages 9 to 32
* Adulthood: Ages 32 to 66
* Early ageing: 66 to 83
* Late ageing: Beyond 83
“The ages 9, 32, 66, and 83 are averages drawn from highly complex data, meaning that there is nothing biologically fixed about these exact numbers. Our research just shows that around those ages there are shifts in how the brain is rewiring,” Mousley says.
WRINKLES IN TIME
The earliest years are certainly the busiest. About half of the quadrillions of neural connections that the brain will eventually form take shape by the age of nine.
Then the real work begins.
The number of connections isn’t as vital as the way those connections are accessed and used, says Mousley. Through our lifetimes, the most commonly used neural links are strengthened as the brain develops hyperfast routes to the most relevant and oft-used information it holds.
For greater efficiency, pathways that are found to be unused may be “pruned”. This is why most of us cannot name, for instance, every former classmate or workmate in an old photo.
“The largest change happens around 32, when the brain shifts into a new gear where it is becoming more segregated,” says Mousley. “From the age of 66 onwards, our research indicates that the brain undergoes subtle shifts, potentially relating more to the speed of change. Around the age of 83, the brain becomes more reliant on multiple links, where earlier there was one.”
What does that mean? Imagine you have a direct bus route to work, but one day that bus stops running, Mousley says. “Now, you need to take two buses and make a transfer. This is similar to what is happening in brains past the age of 83.”
Back to the notion of adulthood, Mousley says, “anecdotally, many people experience their 30s as a period of greater stability, and describe this as the time they ‘got it together’ or finally felt like a ‘real’ adult”.
“For me, this brings up a lot of questions about what is driving this experience,” she adds. “Is it the shift in the brain that drives the feeling of stability? Is it social-cultural expectations that initiate this shift or is it something else entirely? We really don’t know, but hopefully this project helped get us one step closer to understanding these complex developmental processes.”
A GROWN-UP PROBLEM
To what extent are sociocultural expectations shaping this arc of brain development?
“We know environment makes a huge difference to brain development,” says Meera Purushottam, a senior geneticist at India’s National Institute of Mental Health and Neurosciences (NIMHANS).
Casting a look backwards, if average life expectancy during the Bronze Age was 30 or 35, expectation from a young person is bound to have been much different. To understand how such expectation shapes development, “compare a younger sibling with the older one,” Purushottam says. “The child exposed to responsibilities grows up very differently.”
Dr Guy Leschziner, a British neurologist, is intrigued by this variability — between people, and within the same person at different stages. It is the subject of an upcoming book. “The truly interesting thing to me,” he says, “is that our brains change so much throughout our lives that it is almost impossible to consider us one person all through. We each constitute multiple people.”
