We defined co-primary outcomes as the proportion of individuals with age at disorder onset of any and specific mental disorder groups before 14, 18 and 25 years old, and peak age at onset for any mental disorder and for each diagnostic group. Diagnostic groups matched all 19 ICD-11 diagnostic blocks under “Mental, behavioural or neurodevelopmental disorder”, namely neurodevelopmental disorders, schizophrenia-spectrum and primary psychotic disorders, catatonia, mood disorders, anxiety and fear-related disorders, obsessive-compulsive or related disorders, disorders specifically associated with stress, dissociative disorders, feeding or eating disorders, elimination disorders, disorders of bodily experience, disorders due to substance use or addictive behaviour, impulse-control disorders, disruptive behaviour or dissocial disorders, personality disorders and related traits, paraphilic disorders, factitious disorders, neurocognitive disorders, disorders associated with pregnancy childbirth or puerperium. Individual studies adopted different age subgroupings and age ranges. To our knowledge, there is no standard method to pool such varying descriptive statistics of the distribution of a variable (age at onset of mental disorders) when the variable of interest follows a non-normal distribution and/or is heterogeneously censored. Therefore, we have developed an ad-hoc method
