MONDAY, Nov. 19 (HealthDay News) -- A new study from Iceland adds to existing evidence that kids are more likely to be diagnosed with attention deficit hyperactivity disorder if they're among the youngest in their grade at school.

The findings suggest -- but don't prove -- that some children are diagnosed with attention deficit hyperactivity disorder (ADHD) when they're just less mature than their peers.

"Educators and health-care providers should take children's ages in relation to their [classmates] into account when evaluating academic performance and other criteria for ADHD diagnosis," said study author Helga Zoega, a postdoctoral fellow at Mount Sinai School of Medicine in New York City. "Parents can use these findings to help inform their decisions about school readiness for children born close to cutoff dates for school entry."

Debate has erupted in recent years about whether ADHD is overdiagnosed in children who are naturally rambunctious and don't need to be medicated.

Earlier this year, a Canadian study found that more boys aged 6 to 12 were diagnosed with ADHD if they were the youngest in their grade levels: the percentages were 7.4 percent among the youngest boys and 5.7 percent among the oldest.

There also was a gap between older and younger girls, although they're much less likely than boys to be diagnosed with ADHD.

The new study tracked nearly 12,000 children born between 1994 and 1996 in Iceland, where there's also been a debate over the diagnosis of the condition and the use of stimulants to treat it, Zoega said. The researchers looked at the children's test scores at ages 9 and 12, and examined whether they were prescribed drugs for ADHD.

Overall, 740 children -- about 6 percent of the total -- were prescribed ADHD drugs at some point from 2003 to 2009.

Children in the youngest third of their classes were 50 percent more likely than those in the oldest third to be prescribed ADHD medications from ages 7 to 14.

The study also found that "being younger relative to one's classmates affects academic performance throughout childhood," Zoega said.

Richard Morrow, a health research analyst at the University of British Columbia who studies ADHD, said the findings are consistent with those from other countries.

"In the education system, it leads to the question, 'What strategies or resources do we need to help ensure the well-being of all children in the classroom, where children vary in age by up to a year?' " he asked. "Parents need to be aware that if behavioral issues arise for their child, this may be related to their child's relative age in the classroom.

"Similarly, doctors need to consider a child's relative age in school or other settings such as athletics before making a diagnosis or writing a prescription," Morrow added. "Lastly, we may need to revisit how the diagnosis is defined to lessen the risk of inappropriate diagnosis."

The findings appear online Nov. 19 and in the December print issue of the journal Pediatrics.

So what can be done?

"Children behave and perform according to their own maturity level within the classroom," Zoega said. "Being younger relative to one's classmates affects academic performance throughout childhood. When evaluating whether a child has ADHD, this should be taken into account to prevent unnecessary diagnoses and prescribing of stimulants."

More information

For more about ADHD, try the U.S. National Library of Medicine.