Parents who despair over their teenagers' lack of concentration in class, inability to sit still long enough to finish homework or plan ahead, should take solace. Their children are not being lazy or careless – they are hapless victims of neurobiology.

New research has found that teenagers' brains continue developing far longer into adulthood than previously thought. Adolescents may look like young adults but their brain structure resembles that of much younger children, according to the study to be published in the Journal of Neuroscience on Wednesday.

"It is not always easy for adolescents to pay attention in class without letting their minds wander, or to ignore distractions from their younger sibling when trying to solve a maths problem," said Dr Iroise Dumontheil of University College London's Institute of Cognitive Neuroscience, one of the authors of the research. "But it's not the fault of teenagers that they can't concentrate and are easily distracted. It's to do with the structure of their brains. Adolescents simply don't have the same mental capacities as an adult."

Using MRI scans, the brain activity of adolescents were monitored as they tried to solve a problem in their heads while ignoring environmental distractions.

The scans revealed an unexpected level of activity in the prefrontal cortex, a large region at the front of the brain involved in decision-making and multitasking. This indicated that the brain was working less effectively than that of an adult.

"We knew that the prefrontal cortex of young children functioned in this chaotic way but we didn't realise it continued until the late 20s or early 30s," said Dr Sarah-Jayne Blakemore, who led the study. "What we discovered was that the part of the brain needed to complete this sort of process is still very much developing throughout adolescence. This means it continues to do a lot of needless work when making these sorts of decisions."

Chaotic thought patterns are a result, she said, of teenagers' brains containing too much grey matter – the cell bodies and connections which carry messages within the brain. As we age, the amount of grey matter in our brains decreases.

"What our research has shown is that there is simply too much going on in the brains of adolescents," said Blakemore. "The result is that their brain energy and resources are wasted and their decision-making process negatively affected."

Adults, on the other hand, have less grey matter, said Blakemore. "This means that neural transmissions travel more efficiently between brain cells, so the brain works more effectively."