People do not become fully “adult” until their 30s, according to brain scientists.
The rocky path to adulthood also varies from person to person, with some individuals making the transition faster than others, say experts.
New insights into how the brain is wired and reshaped throughout much of a person’s life have major implications for society, they claim.
Currently the judicial system in the UK recognises a person of 18 as being a mature adult.
In fact someone of this age is still undergoing tumultuous changes in the brain that can affect behaviour and susceptibility to mental health disorders, research has shown.
Processes that involve boosting the conductivity of nerves, building neural networks and “pruning” away unwanted connections begin in the womb and continue for decades.
Upheaval in the brain is thought to account for the notoriously difficult behaviour of adolescents, typified by comedian Harry Enfield’s character Kevin the Teenager.
Professor Peter Jones, from Cambridge University, told journalists at a press briefing in London: “What we’re really saying is that to have a definition of when you move from childhood to adulthood looks increasingly absurd.
“It’s a much more nuanced transition that takes place over three decades.”
He added: “I guess systems like the education system, the health system and the legal system make it convenient for themselves by having definitions.”
Prof Jones is one of a number of international leading experts attending a neuroscience meeting hosted by the Academy of Medical Sciences in Oxford.
He suspected that despite the legal definition of adulthood, experienced judges recognised the difference between a 19-year-old defendant and a “hardened criminal” in his late 30s.
“I think the system is adapting to what’s hiding in plain sight, that people don’t like (the idea of) a caterpillar turning into a butterfly,” he said.
“There isn’t a childhood and then an adulthood. People are on a pathway, they’re on a trajectory.”
American colleague Professor Daniel Geschwind, from the University of California at Los Angeles, stressed the degree of individual variability in brain development.
He pointed out that for practical reasons, education systems mistakenly tended to focus on groups, rather than individuals.
Prof Geschwind added: “These are larger questions that go beyond the science.
“If one takes one thing away it’s that there are individual trajectories, (and) that development takes place over decades. But this varies from individual to individual.”
A key topic at the meeting was research into serious mental disorders such as schizophrenia.
Once thought to have a purely genetic cause, schizophrenia and other psychotic conditions are now known to arise from a complex interplay of genes and environmental influences.
Schizophrenia is typically diagnosed in older teenagers and the risk of developing the disease falls dramatically from the late 20s onwards.
The incidence pattern is thought to be linked to brain development. Once the brain has sorted out its circuits and finally “matured”, the risk of psychosis greatly diminishes.
Prof Jones said studies had shown that people living in cities, especially poor and migrant populations, faced an increased risk of serious mental disorders.
Urban living produced a “potent cocktail” of environmental influences that could affect the developing brain, he argued.
He said: “Being a migrant isn’t specifically about one particular group, it’s about being a minority within a majority.
“It’s probably to do with having to live constantly on the alert. I mean a low level of vigilance that minorities experience when they’re living in host communities.”
Prof Jones referred to one study focusing on a population living on the outskirts of Paris, France, which suggested that being part of a minority group tripled the risk of schizophrenia.
An on-going investigation in the US, the Adolescent Brain and Cognitive Development study, is following the progress of almost 12,000 children aged nine and 10 looking for early signs of future mental problems.