At least two authors (MO, ER, LS, MS, all MDs) independently searched PubMed and Web of Science using the following terms: “age at onset” (topic) and “mental disorder” (topic), plus “birth cohort” (topic) and “mental disorder” (topic), plus “incidence” (topic) and “mental disorder” (topic), without restriction to the type of mental disorder. Additionally, reviews and reference lists of included studies were manually searched. The literature was searched from database inception until 16/05/2020. Hits were first screened at the title/abstract level, then full texts of the remaining articles were assessed, recording reasons for exclusion (as per the criteria noted below).
