Relative Age Effects on Attention-Deficit/Hyperactivity Disorder Symptoms and Educational Achievement: A Longitudinal UK Cohort Study

Objective

Being among the youngest in a school class increases the risk for worse educational outcomes and attention-deficit/hyperactivity disorder (ADHD) symptoms, but questions remain about the nature and persistence of such effects. We investigated this “relative age effect” on educational achievement at age 15 to 16 years and on ADHD symptoms from age 7 to age 21 years. Furthermore, we examined whether being young-in-class is linked to a greater reduction in ADHD symptoms from childhood to adulthood and a lower genetic propensity to ADHD.

Method

We identified 3,928 young-in-class and 4,580 old-in-class participants from the Twins’ Early Development Study. Educational achievement was measured with mathematics and English examination grades at age 15 to 16 years, and ADHD symptoms were measured using 2 different scales and different raters, from age 7 to 21 years, with effects tested using regression.

Results

A relative age effect emerged for English but not mathematics examination grades, and for the majority of parent and teacher ratings on ADHD symptoms, most consistently in middle childhood. Being young-in-class was associated with a greater reduction in parent-rated ADHD symptoms from childhood to adulthood when measured with a brief scale, but the comparable result from a longer scale was non-significant (after multiple testing correction). No interaction emerged between relative age and ADHD polygenic scores.

Conclusion

Our results emphasise the need to improve support for the children who start school younger than most, and to ensure that developmental comparisons take children’s precise age into account. Future research would benefit from in-depth analyses of individual trajectories and their variability among the young-in-class children.

Plain language summary

Young-in-class children are at increased risk for worse educational outcomes and attention-deficit/hyperactivity disorder (ADHD) symptoms, but their persistence is unclear. This study examined data from a large longitudinal UK community sample (3,928 young-in-class and 4,580 old-in-class) to identify age effects on educational achievements and ADHD symptoms. Results showed that young-in-class children are more likely to be rated with ADHD symptoms by both parents and teachers, especially in middle childhood, and have lower English exam grades at age 15 to 16.These results emphasize the need to improve support for children who start school younger than most and ensure that developmental comparisons take children’s precise age into account.