AS A woman in my mid-30s, I look very much like a grown-up. I have a career, a home and a husband; I have deliberated over and purchased a dishwasher, a washing machine and a fridge. Judge me by my white goods, and I tick all the boxes.
But when, one Saturday, I opened my kitchen bin to find the lid heaving with maggots, I felt totally helpless. So I called my mum. It was a crushing realisation: I may look like a grown-up, but I didn鈥檛 feel like one. It got me thinking, if a house and a spouse don鈥檛 make an adult, what does?
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If you go by traditional milestones, people in the West seem to be growing up later than ever. The age of the average first-time home buyer in the UK has risen by seven years over the past six decades. In the US, the over the same period. And the proportion of women in Australia having their first child when over the age of 30 more than doubled between 1991 and 2019.
Yet according to UK law, the answer is straightforward: an adult is anyone who is 18 or older. This age determines so much, from whether you can vote and how you can access the National Health Service (NHS) mental health services, to whether you can get a drink in a pub. It is hard to overstate the impact this definition can have on the lives of young people, which is why I wondered whether it needed a rethink. At what age do we really become adults?
While writing my book , I interviewed various people whose stories highlight why it is so important to better understand our transition to adulthood. Boru, for instance, told me his mental health had plummeted in his mid-teens: he refused to go to school, wouldn鈥檛 eat, became addicted to drugs and attempted suicide. At 17, he was admitted as an inpatient to the adolescent unit of a psychiatric hospital and stayed there for several months, with later re-admissions for brief periods. He had access to a psychotherapist, took part in creative group activities and began to feel he was recovering. Then he turned 18.
While there are some mental health services in England that treat young people up to the age of 25, this isn鈥檛 the norm, and it wasn鈥檛 the case for Boru. He couldn鈥檛 continue to see the same therapists he had come to know and trust, couldn鈥檛 return to the same hospital he felt safe in. Boru was diagnosed with drug-induced psychosis and admitted to a psychiatric hospital again 鈥� this time in the adult department. 鈥淚 was being told: 鈥榊ou are an adult now, and we鈥檙e going to treat you as such,'鈥� he says. He was terrified: 鈥淚 was shitting myself. Everyone鈥檚 older than you, and you鈥檙e the little kid.鈥�
Many neuroscientists, such as at the University of Cambridge, push back at such a strict definition of adulthood. 鈥淭he idea that the brain suddenly becomes mature [at 18] isn鈥檛 true,鈥� she says. 鈥淭he brain is not a uniform piece of tissue, it is made up of different regions, which each develop at different rates, and different people鈥檚 brains mature at different ages.鈥�
The developing brain
Blakemore and her colleagues found that well past the age of 18, through to our late 20s. These include the prefrontal cortex, which is key to decision-making, understanding other people and risk-taking; the temporal cortex, which is involved in language and memory; and in the parietal cortex, involved in movement and navigation.
Another aspect of brain development, called pruning, also continues past the age of 18. We are born with many more connections, or synapses, between neurons in the brain than we need. At around 8 months old, a process of elimination begins, whereby useful synapses are strengthened and others are removed. This streamlining of our neural architecture is fundamental to our cognitive functions, and studies show that , sometimes even into our 30s.
We might also consider the development of white matter in the brain to be important to when we become adults. White matter is made up of long fibres called axons, which are covered in a fatty substance called myelin. This increases in volume as we grow up and is associated with faster transmission of electrical signals 鈥� and therefore information 鈥� around the brain. Studies show that this .
So your brain is maturing well past 18. And, one could argue, it never really stops. No matter how old you are, : this phenomenon is known as neuroplasticity. 鈥淭here is no age limit to plasticity,鈥� says Blakemore. 鈥淲e are constantly evolving and learning and adapting and changing.鈥�
So if studying the brain can鈥檛 provide us with an answer, where else can we look?
, Massachusetts, has been researching when young people begin to feel like adults. He made his mark in developmental psychology 20 years ago by called 鈥渆merging adulthood鈥�.
鈥楾he big three鈥� adult skills
Arnett has conducted dozens of studies, interviewing hundreds of people across the US, asking them whether they felt like adults and what that meant for them. He says he took great care to include people with a range of educational qualifications from varied socioeconomic backgrounds. Arnett found that most people began to feel like an adult somewhere between the ages of 18 and 29: early in this period, he explains, very few people feel like adults, but by the end of it, almost everybody he asked replied yes, they did. They would then usually mention responsibility, which links what he calls 鈥渢he big three鈥�, the core elements that constitute adulthood. According to the vast majority of his interviewees, these were the capability to take care of themselves, being able to make their own decisions and having financial independence.
Over the three decades that Arnett has been asking people in the US this question, the answer hasn鈥檛 changed, he says. It isn鈥檛 a global answer, however. In 2014, Arnett鈥檚 colleague , now at Springfield College, Massachusetts, travelled to China . The participants, aged 18 to 29, completed a questionnaire of markers for adulthood, then 15 were interviewed on their understanding of this transition. The responses were subtly but significantly different from those in the US. The 鈥渂ig three鈥� criteria of adulthood were orientated more around taking responsibility for others, rather than for oneself. They included learning to care for parents, settling into a long-term career and feeling capable of caring for children. Despite the global disparity in the detail, by Arnett and Zhong鈥檚 definition, most people around the world feel like adults by age 29.
But when it comes to measures of adulthood, the big three criteria from these studies paint only a partial picture. You could argue that being an adult is also about how we tolerate and make sense of our emotions.
This is the kind of growing up that I see a lot in my work as a psychodynamic psychotherapist in the NHS. As young infants, we defecate or scream or lash out to deal with our emotions. As we transition into becoming adults, we develop more and more sophisticated means to digest feelings. This may not be a clear-cut measure of adulthood, but it has meaning as a measure of the growth of an internal capacity to process the world in a more mature way.
Emotional intelligence
Such 鈥渆motional intelligence鈥� appears around age 4 and develops at different rates in different people. There is no consensus on when your emotional intelligence stops developing, if it ever does. However, a study in China did show emotional intelligence was directly related to the age at which a person experiences high affective well-being, which is defined by a balance of positive and negative emotions 鈥� a characteristic some might associate with settling into adult life.
When considering the transition to adulthood we should also consider reproduction, a common signifier for many people of becoming a 鈥減roper鈥� grown-up (see 鈥淲hen do animals grow up?鈥�).
To find out whether reproduction makes its mark on the maturing brain, at Amsterdam University Medical Centre in the Netherlands scanned the brains of people who wanted to become pregnant before and after giving birth, then . The brain changes after pregnancy were so strong and consistent that a computer algorithm could detect all of the people who had been pregnant based only on their scans. 鈥淚t was truly exceptional,鈥� says Hoekzema.
Baby brain
The structural changes were most apparent in brain regions that play a role in theory of mind, which is the capacity to understand someone else鈥檚 psychological state, emotions and thoughts. 鈥淚t鈥檚 an ability that plays an important role in parenting,鈥� says Hoekzema. 鈥淚t seems that these structural changes help prepare a woman鈥檚 brain to optimally respond to cues of her infant and help prepare her for her new role,鈥� she says. Studies showed .
There are strong indications that pregnancy entails a further maturation of the brain, or at least a further specialisation that benefits the next phase of life, says Hoekzema. You don鈥檛 have to grow up before you become a mother, it seems; pregnancy itself causes changes that help you to mature.
An alternative way to think about when we become adults is to consider when our brain starts to decline. Perhaps this might give some clues as to upper limits? Not so fast. at the University of Cambridge studies middle age. In neurological terms, he says, this period, between 40 and 60, was historically thought of as a time where nothing much happens. 鈥淧eople neglected it for the more sexy and exciting parts of life,鈥� he says.
Now, though, studies have shown that , even as others are still maturing. Brain scans have shown this deterioration most clearly in the frontal lobes, which we use to make plans and solve problems. Other studies of executive functions, including things like working memory 鈥� the ability to hold information in mind 鈥� and planning, show that .
Cognitive decline
Thankfully, this midlife decline in cognitive skills will be barely noticeable in everyday life, says at the University of Kent in the UK. But it does underlie the onset of significant difficulties that people experience in older age, she says. 鈥淯nderstanding that these changes begin as early as our 30s is important for us to begin looking for precursors and consequences in the midlife period.鈥�
This counter-intuitive revelation 鈥� that our brains can grow and deteriorate in tandem throughout our 30s and onwards 鈥� sums up the complexities involved in pinning down the onset of adulthood. Even if we consider all of the valid definitions of adulthood, it seems to occur somewhere between puberty and our late 40s.
For some, adulthood may arrive even later. Pog was 90 years old when I spoke to her for my book. I asked her if she was a grown-up. She laughed and said: 鈥淚鈥檓 quite shocked really. I truly do not consider that I have grown up.鈥� I asked her if she feels she will manage to finish growing up before the end of her life and she replied, thoughtfully: 鈥淚 hope not.鈥�
For people like Boru, subject to an arbitrary definition of adulthood, a more nuanced and scientific view of when we become a grown-up would be life-changing. For me, Pog鈥檚 answer has the most meaning for when we finish growing up: the lucky ones never do.
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When do animals grow up?
In the animal kingdom, it has long been assumed that it is the capacity to reproduce that differentiates adults from their young. Yet in 2020, , a cardiologist and evolutionary biologist at Harvard University, overturned that assumption. She and her colleague , an animal behaviourist and author, spent five years studying the transition to adulthood in wild animals around the world, from grey wolves and humpback whales to African elephants. They concluded in their book, , that the ability to reproduce isn鈥檛 synonymous with adulthood.
For many species in the wild, puberty 鈥� the physiological process of becoming reproductively mature 鈥� is just the beginning. 鈥淭hese young animals lack adult skills in many areas. They may lack courtship skills because they haven鈥檛 learnt mating dances, or they may not have social skills to acquire status,鈥� says Natterson-Horowitz.
She points out that it was widely believed that newly adult penguins, having shed their soft brown down and now sporting their black and white feather tuxedos, would dive into the water and head off to create families of their own. But Natterson-Horowitz and her colleagues discovered that if there isn鈥檛 enough food or if the penguins鈥� hunting skills aren鈥檛 sufficiently developed, those in a number of species return to their parents, who continue to feed them 鈥� what researchers call 鈥渆xtended parental care鈥�. 鈥淭hey鈥檝e left home, but come back because they can鈥檛 yet make it on their own,鈥� she says.
Over five years of conducting large systematic reviews of adolescence across species and fieldwork observing animal adolescents in natural settings and sanctuaries, Natterson-Horowitz and Bowers arrived at a new definition of adulthood based not on an animal鈥檚 capacity to reproduce, but on the mastery of four key competencies: staying safe, navigating social hierarchies, sexual communication and leaving the nest to care for itself. 鈥淪afety. Status. Sex. Self-reliance,鈥� says Natterson-Horowitz. Master all four and you have an adult animal. Could the same be said for humans? (See main story.)
