Relative Age in School and Initiation of Speech Therapy in Children.

Importance: In most countries, children in the same grade can be up to 12 months apart in age, which represents a considerable relative age difference, particularly in the early years. A relative age effect has been frequently reported in the diagnosis and treatment of attention-deficit/hyperactivity disorder and, more rarely, in the diagnosis of other neurodevelopmental disorders. The relative age effect has never been studied for speech therapy, which is frequently prescribed to treat specific language and learning disorders. Objective: To quantify the association of relative age with the initiation of speech therapy. Design, Setting, and Participants: This cohort study used data from the French National Health Data System and included all children born in France between 2010 and 2016 from September of the year of their 5th birthday until July of the year of their 10th birthday or July 31, 2022 (end of study). Exposure: Children were categorized according to month of birth. Main Outcomes and Measures: Risk of speech therapy initiation was estimated using Cox models adjusted for other known risk factors. Results: In total, 4 188 985 children (mean [SD] age, 5.2 [0.3] years; 50.8% girls) were included. Speech therapy was initiated for 692 086 children (incidence rate, 53.1 per 1000 person-years, 54.5% boys) during a mean (SD) follow-up of 3.1 (1.6) years. Among children of the same schooling level, the risk of initiating speech therapy steadily increased according to quarter and month of birth, with adjusted hazard ratios of 1.51 (95% CI, 1.50-1.52) for children born in the last quarter vs the first quarter and 1.64 (95% CI, 1.62-1.66) for children born in December vs those born in January. The relative age effect observed for speech therapy was of the same magnitude as that observed for methylphenidate in attention-deficit/hyperactivity disorder (positive control outcome). No relative age effect was observed for the initiation of desmopressin for nocturnal enuresis (negative control outcome). Conclusions and Relevance: This cohort study found that among children in the same grade, an age difference of a few months was associated with the frequency of initiation of speech therapy. These findings may be attributable to a mismatch between the expectations of the school system and the relative age and level of maturity of the children or earlier identification of neurodevelopmental disorders in the youngest children in each age group.