Kids with attention deficit hyperactivity disorder (ADHD) may delay the maturing of the brain for as long as three years, according to a new study published in the journal Proceedings of the National Academy of Sciences.

The study led by Dr. Philip Shaw, a child psychiatrist at the National Institute of Mental Health and colleagues found that in kids with ADHD, the prefrontal cortex of the brain matured about three years later, around the age of 10.5 years compared to kids without the condition.

This region of the brain is responsible for focusing attention and suppressing inappropriate thoughts and actions, which are disrupted in kids with ADHD.  This may explain why children with the disorder could not focus as well as normal children.

In addition, the study found the motor cortex that is responsible for making different movements in the body actually matured faster in children with ADHD than children without the condition.

The researchers suggest that these two findings may be responsible for the restlessness and fidgety symptoms commonly seen in children with ADHD.

The study involved 446 children and the researchers examined 40,000 sites of the brain cortex with magnetic resonance imaging scans.

ADHD affects 3 and 5 percent of children, or approximately 2 million children in the United States, according to The National Institute of Mental Health.

A recent study published in September in The Lancet found a variety of common food dyes and the preservative sodium benzoate, which is widely used in processed foods and beverages. The findings suggest that avoiding these artificial ingredients can minimize ADHD symptoms in children.

In addition to food additives, the adjustable environmental risk factors for ADHD include drinking alcohol and smoking during pregnancy, brain injury and use of sugars, according to the NIMH.

The results of the current study did not suggest any remedy for the late development of the brain nor reveal any cause for ADHD. But the message seems to be that ADHD may cause some consequences and parents may need to do something about it.

Larry Seidman, professor of psychology at Harvard Medical School was quoted by ABC News as saying "The use of the word 'delay' could be misinterpreted to mean that children with ADHD will 'catch up.'

"As of now, we don't have evidence that they catch up in their brain development," he added.