A growing number of older U.S. children are being diagnosed with attention deficit hyperactivity disorder, while diagnoses among younger children have held steady, government researchers said on Wednesday.

The report by the Centers for Disease Control and Prevention found ADHD diagnoses among children aged 12 to 17 increased by an average of 4 percent a year from 1997 to 2006. The researchers found no significant change in the percentage of children aged 6 to 11 diagnosed with ADHD over the same period.

The researchers used statistics from a national health survey that included data on 23,000 children aged 6 to 17 gathered in 2004, 2005 and 2006.

Overall, they found that nearly 5 percent of children aged 6 to 17 had ADHD, a condition that often becomes apparent in preschool and early school years. Children with ADHD have a tougher time controlling their behavior and paying attention.

The researchers estimated that as of 2006, a total of 4.5 million school-aged children - those aged 5 to 17 - had been diagnosed with ADHD.

Boys were more than twice as likely as girls to have ADHD, confirming similar findings from other studies.

The study also found that Hispanic children were less likely than non-Hispanic black or white children to have ADHD.

Children with ADHD were more likely than others to have contact with a mental health professional, to use prescription drugs and have frequent health-care visits.

The researchers did not say why older children were being diagnosed at a higher rate than younger children, but suggested it may be that older children had more chances of being evaluated and diagnosed than younger children.

ADHD is marked by restlessness, impulsiveness, inattention and distractibility that can interfere with a child’s ability to pay attention in school and maintain social relationships.

The typical treatment for ADHD is with a drug like Ritalin, or methylphenidate, a stimulant intended to lower impulsiveness and hyperactivity and boost attention.

CHICAGO (Reuters)