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 for attention-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.’