The aim of this article is to review recent epidemiological research on age-of-onset of mental disorders, focusing on the WHO World Mental Health surveys. Median and inter-quartile range (IQR; 25th-75th percentiles) of age-of-onset is much earlier for phobias (7-14, IQR 4-20) and impulse-control disorders (7-15; IQR 4-35) than other anxiety disorders (25-53, IQR 15-75), mood disorders (25-45, IQR 17-65), and substance disorders (18-29, IQR 16-43). Although less data exist for nonaffective psychosis, available evidence suggests that median age-of-onset is in the range late teens through early 20s. Roughly half of all lifetime mental disorders in most studies start by the mid-teens and three quarters by the mid-20s. Later onsets are mostly secondary conditions. Severe disorders are typically preceded by less severe disorders that are seldom brought to clinical attention. First onset of mental disorders usually occur in childhood or adolescence, although treatment typically does not occur until a number of years later. Although interventions with early incipient disorders might help reduce severity-persistence of primary disorders and prevent secondary disorders, additional research is needed on appropriate treatments for early incipient cases and on long-term evaluation of the effects of early intervention on secondary prevention.