Association of parental age with developmental vulnerability in early childhood: a population-based cohort study

Background With delayed childbearing increasingly prevalent in the Western Pacific, its life-course implications for early childhood development merit closer examination. This study evaluated the associations between parental age and child multidomain developmental vulnerability (MDV) at 36 months, treating MDV as a population-level screening construct rather than a clinical diagnosis to differentiate between potential physiological and socioeconomic pathways. Methods Data from the Taiwan Birth Cohort Study (n=18 171) were analysed. Developmental status was assessed across five domains (gross/fine motor, language, sociocognitive and personal-social). MDV was defined as ≥2 domains below the cohort-specific 10th-percentile threshold. Directed acyclic graph-guided modified Poisson regression with sequential modelling was used to estimate adjusted relative risks (aRRs), addressing potential overadjustment bias. Results Of the 18 171 pairs, the maternal age distribution was 17.9% for 20–24 years, 69.6% for 25–34 years, 11.0% for 35–39 years and 1.5% for ≥40 years. Maternal age showed a reverse J-shaped association with MDV. Among younger mothers (20–24 years), elevated risk persisted (aRR 1.23, 95% CI 1.10 to 1.39) after adjustment for socioeconomic and neonatal factors. In older mothers (≥40 years), the association attenuated (aRR 1.21 to 1.13) after accounting for paternal age and perinatal mediators. Risk was highest for parental dyads where both were aged 20–24 years (aRR 1.31, 95% CI 1.07 to 1.60), whereas advanced paternal age was statistically associated with lower vulnerability scores in the personal-social domain (aRR 0.80). Conclusions These findings are consistent with a dual-track framework: developmental risk in younger parents may be partially explained by socioenvironmental determinants, while risk in older parents may reflect physiological pathways. Precision public health strategies could prioritise risk-stratified psychosocial support for younger families alongside clinical monitoring for older parents to mitigate developmental inequalities and improve early childhood outcomes.