This study has several limitations. First, data were too sparse to calculate country-specific estimates. However, the database included many countries worldwide, and, therefore, the estimates provided are largely representative of the global general population. Second, the 95% confidence interval for some estimates were broad since analyses for some disorders were based on few studies. Third, data were heterogeneous, making traditional meta-analytic techniques unfeasible. We, therefore, applied advanced meta-analytic techniques, including bootstrap methods that helped to address this shortcoming and provide unified outcomes across all mental disorders. Fourth, our quality assessment was non-standardised because appropriate validated measures were not available. However, we per-protocol included the highest-quality studies to study age at onset according to previous recommendations [30]. Fifth, the definition of age at disorder onset was heterogeneous; we addressed this issue via sensitivity analyses. Sixth, we were not able to account for and differentiate among comorbid and standalone diagnoses. Sixth, we were not able to account for differences across regions. Finally, caution is needed when comparing peaks (e.g., between symptoms and diagnosis), in particular when the peak curves are different, and when comparing median age at onset across disorders (e-Table 5).
