This raises questions over when people should legally be considered 'mature'

Evidence suggests that the brain changes significantly until at least 30

It may come as no surprise to growing numbers of parents whose ‘boomerang’ children in their twenties have returned to live in their family home.

While they may be old enough to drive, vote, marry and join the army people younger than 30 are still not fully mature, say experts.

This raises questions over whether criminal offenders under 30 should be treated differently to their older counterparts.

The key changes in the brain that occur from adolescence into one’s 20s and 30s is a thinning of the grey matter, and a thickening of white matter

HOW THE BRAIN CHANGES WITH AGE One of the difficulties in establishing when the brain is mature is finding an adult brain that has finished maturing ‘as a point of reference.’ The key changes in the brain that occur from adolescence into one’s 20s and 30s is a thinning of the grey matter, and a thickening of white matter. The increase in white matter represents a growth in connectivity between different brain regions. The thinning of the grey matter represents what scientists call ‘pruning’ – a specialisation in the set-up of the grey matter which makes it more efficient. The part of the brain that keeps growing is the prefrontal cortex. This area of the brain – which is located at the front of the head - is involved in a host of important factors – including attention, complex planning, decision making, impulse control, logical thinking, organised thinking, personality development, risk management and short term memory. But because the brain keeps on changing, even saying that change has largely levelled off by 30 is a problem. Advertisement

For the most sophisticated processes, such as attention, decision making, and taking risks, our brain only starts to settle down in our fourth decade.

But it is still changing for many years after we legally become adults.

Both in Britain and the US issues of whether someone is ‘mature’ are key questions for policy makers – and the criminal justice system.

For instance, a criminal justice report from the House of Commons Select Committee recommended ‘young adults’ aged 18-25 should be treated differently from older criminals as their brains are still maturing.

The report noted most young people involved in crime stop by the age of 25.

At the same time there are increasing demands in England and Wales to grant the vote to 16-year-olds.

Dr Somerville writing in the journal Neuron said: ‘When considering whether an individual brain can diagnose someone as mature or immature, neuroscientists have deep concerns about trying to make those kinds of inferences.

‘The very idea that we could come up with some number that would encompass all of the complexity involved in brain development is a challenge.

‘While there are decades of evidence that adolescents behave differently from adults, the age of 18 doesn't have any biological magic to it.’

Dr Somerville, said that on the basis of research the answer to the question when the average brain reaches a ‘threshold of maturity’

‘From what I’ve reviewed above, the answer might lie sometime between “the 30s” and “never”.

‘This range is remarkably late, given that arguments about reaching maturity tend to focus on the brains and behavioural profiles of individuals in their late teens and early twenties.’

One of the difficulties in establishing when the brain is mature is finding an adult brain that has finished maturing ‘as a point of reference.’

The key changes in the brain that occur from adolescence into one’s 20s and 30s is a thinning of the grey matter, and a thickening of white matter.

The increase in white matter represents a growth in connectivity between different brain regions.

While they may be old enough to drive, vote, marry and join the army people younger than 30 are still not fully mature, say experts. This raises questions over whether criminal offenders under 30 should be treated differently to their older counterparts

The thinning of the grey matter represents what scientists call ‘pruning’ – a specialisation in the set-up of the grey matter which makes it more efficient.

The part of the brain that keeps growing is the prefrontal cortex.

This area of the brain – which is located at the front of the head - is involved in a host of important factors – including attention, complex planning, decision making, impulse control, logical thinking, organised thinking, personality development, risk management and short term memory.

But because the brain keeps on changing, even saying that change has largely levelled off by 30 is a problem.

Dr Somerville said that as the brain keeps changing throughout adult life, this ‘challenges the very notion that the brain reaches a steady adult referent that we can concretely call “mature”.

She said that one of the key differences between adolescents and people in their 20s and older is how distractable they are.

This could mean that an adolescent might be considered ‘mature’ enough to engage in simple behaviour relating to achieving a goal in ‘neutral, non-distracted circumstances.’