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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.