Background: Observational research shows that a child's relative age within their school year ('relative age') is associated with educational attainment and mental health. However, previous studies have only examined a small number of outcomes and evidence of the persistence of effects into adulthood is mixed. We conducted a hypothesis-free investigation of the effects of relative age. Method: We used a regression discontinuity design and an instrumental variable (IV)-pheWAS in the UK Biobank (participants aged 40-69 years at baseline), using the PHESANT software package. We created two IVs for relative age: being born in September vs. August (n=64 075) and week of birth (n=383 309). Outcomes passing the Bonferroni-corrected P value threshold for either instrument were plotted to identify those displaying a discontinuity at the school year transition. Results: We found 21 traits associated with at least one of the instruments (P value below the Bonferroni threshold). Of these, 13 showed a discontinuity at the school year transition. These included previously identified effects including those with a younger relative age being less likely to have educational qualifications and more likely to have started smoking at an earlier age. We also identified a novel potential effect of a younger relative age in school year causing a better lung function as adults. Conclusion: Educational policy should address educational inequality due to relative age. Further research should seek to replicate our identified effect on lung function in different populations, and investigate the mechanisms through which this effect may act.

The authors have declared no competing interest.

This research was funded in whole, or in part, by the Wellcome Trust [grant number 226909/Z/23/Z]. For the purpose of open access, the author has applied a CC BY public copyright licence to any Author Accepted Manuscript version arising from this submission. M.A.dL is funded by the Wellcome Trust [grant number 226909/Z/23/Z]. M.A.dL, K.T. and L.M. work in a unit that receives support from the University of Bristol and the UK Medical Research Council [grant numbers MC_UU_00011/1, MC/UU/00011/3]. N.M.D is supported by the Norwegian Research Council [grant number 295989].

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