ADHD Diagnosis Is Twice As Likely For Youngest Kids In Class, Study Says

The youngest kids were also 80% more likely to be prescribed ADHD medication.

Young child wearing backpacking outside of school pulling away while his dad tries to control him.
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For parents of kids whose birthdays fall just before the school year cutoff, choosing when to let your children start school is a complicated decision. That cutoff, for many school systems in the United States, is Sept. 1. So what are kids with July or August birthdays to do? Are they better off if their parents enroll them the first year they're technically eligible? Or are they more likely to thrive if they wait a year, making them the oldest kids in their class instead of the youngest? There isn't a one-size-fits-all answer, but recent research points to a key consideration: The youngest kids in a class are more likely to be diagnosed with ADHD and prescribed medication for it than their older peers.

For the recent study, researchers examined data on all Norwegians born between 1989 and 1998, which includes more than 488,400 people. They then analyzed trends in the medications prescribed to these individuals between ages 10 and 23 in relation to when they were born in the year. (In Norway, children born in November and December are typically the youngest in their class, and those born in January and February are usually the oldest.)

At ages 10 to 14, kids born at-term in the November-December group had 80% higher odds of being prescribed ADHD medication compared to those born in January-February, says study author Christine Strand Bachmann, M.D., a Ph.D. candidate in the nursing and public health department at the Norwegian University of Science and Technology and a pediatrician at St. Olav’s University Hospital in Trondheim, Norway.

"For every increase in birth month [grouping], we found an increase in prescription rates," Bachmann says. These prescription discrepancies decreased when the at-term children were in high school.

These trends aren't isolated to Norway. "These relative age effects for ADHD medication have been shown from all over the world...from the Nordic countries, Asia, Western Europe, Canada, and the United States," Bachmann says. Only Denmark seems to be an outlier, which may be because 40% of the children born between October and December chose to delay the start of schooling by a year, Bachmann says. Or, as the Denmark study authors note, it could be because many people with ADHD in the country forego medication.

For countries such as Norway and the U.S. where researchers have noticed a correlation between relative age and the odds of being prescribed ADHD meds, some of the youngest children may be getting misdiagnosed. For them, “maybe the problem is comparison with the older mates,” says Bachmann.

“Put simply, it looks like we’re medicating the most immature children because we’re comparing them to their oldest classmates, who are a whole year older,” Bachmann said in a press release. “This shouldn’t be the basis for receiving an ADHD diagnosis.”

However, it may also be true that some of the youngest children do have ADHD, and their neurodivergence is simply being identified at an earlier stage, NPR notes.

The researchers also looked at children who were born pre-term, before the 37th week of pregnancy, because they’re more likely than children born at-term to be prescribed ADHD medication. "For the preterm population, we found smaller effects," Bachmann says. The preterm November-December group had 40% higher odds of being prescribed ADHD medication than the preterm January-February group. But instead of this discrepancy leveling out during the children's high school years, the effect lasted through them.

If your child is slated to be one of the younger kids in class and you have the option of delaying a year, Bachmann recommends giving it serious consideration. Do they seem truly ready to start school? She also notes that kids who are young for their grade and were born preterm may have been put into a younger grade anyway if they had been born at-term, so it may be especially beneficial if they delay the start of school by a year.

The Child Mind Institute describes in detail factors to consider in this decision, which include social-emotional and language development, along with self-regulation skills and ability to follow directions. “It doesn’t matter if your child is academically ready. If they aren’t ready to navigate a classroom, they should be held back,” kindergarten teacher Donna Pollack Sacks told the Child Mind Institute.

Bachmann adds that signs a child may do best by delaying school by a year include if they prefer to play with younger kids and seem less mature than other children their age, or if they struggle to sit still during seated activities.

"Often, parents have this unique 'stomach feeling,'" that their child may not be ready, Bachmann says. Trust your gut.

And if your school thinks your relatively young child should be evaluated for ADHD, don’t be dismissive. "I think it's saying something that the school sees that the child struggles,” Bachmann says. She recommends that parents get involved with the ADHD evaluation process and share their insights about how the child behaves at home and in other contexts. "It's in the diagnostic criteria that one should see the symptoms and traits at all areas — so not just in school, but also at home and in other places," she says.

This process may also identify areas where adjustments could be made to help the younger child be more successful in the classroom, she adds. In other words, even if they don’t get a diagnosis, the evaluation process can help identify how to best meet your kid’s needs, even if they only seem restless or inattentive because they’re on the younger side.