The youngest kids in the class may have a tougher time with academics and behavior, a new study found.

Icelandic elementary school students in the bottom third of their fourth-grade class for age were almost twice as likely to score low on math and language arts standardized tests, study author Helga Zoëga of Mount Sinai School of Medicine in New York City and colleagues found.

They were also 50 percent more likely to be prescribed stimulants forattention-deficit hyperactivity disorder (ADHD) by seventh grade than the oldest kids in their class, the group reported in the December issue of Pediatrics.

"Birthday cutoffs for school entry necessarily lead to an age span of at least 12 months within a classroom," they noted. "At age 5, this span accounts for 20 percent of the child's age and presents a difference in maturity and performance between the youngest and the oldest child in class."

Since the gap appeared to persist through age 14 in the study, "this should be taken into account when evaluating children's performance and behavior in school to prevent unnecessary stimulant treatment," to make sure kids aren't getting a lifelong label merely because of their immaturity relative to older peers, Zoëga's group argued.

Parents also might want to consider these results in deciding when to start their kids in school, they suggested.

Given controversy on the subject, the investigators used national databases from Iceland to pull together the evidence on standardized test results and filled psychotropic drug prescriptions among all 11,785 children tested at age 9 to 12 there.

The youngest students in fourth grade, born in September to December, came in 10 percentiles lower on average on standardized tests in both math and language arts than the oldest third, born in January to April.

By seventh grade, the difference was smaller but still substantial. Averages were seven percentiles lower for the youngest third versus the oldest third in both areas.

Prescription of stimulant medication for ADHD as a measure of behavioral difficulties showed higher rates for younger kids. The rates rose from 5.3 percent in the oldest third to 5.6 percent in the middle group born in May to August to 8 percent for the youngest third.

Both standardized test scores and ADHD prescriptions showed significant effects of age strata for girls as well as boys.

The researchers cautioned that these measures may only partially predict long-term academic and psychiatric outcomes.

Nor could the study rule out possible undertreatment of ADHD in the oldest kids as the mechanism rather than overtreatment in the youngest, the investigators noted.

Generalizability may also be an issue, as Iceland's universal healthcare as well as other local educational, structural, and cultural factors may have influenced the results, they acknowledged.

They pointed to a large U.S. study showing double the risk of diagnosis or treatment for ADHD in children born in the 4 months prior to the school eligibility age cutoff.