"Early onset" refers to the occurrence or beginning of something, such as a disease or condition, happening at a younger age than usual or expected. Full definition
Family structure influences the risk of early onset of sexual intercourse. (link.springer.com)
Early - life adversity - induced long - term epigenetic programming associated with early onset of chronic physical aggression: Studies in humans and animals. (cambridge.org)
It is usually found in early onset cases, before the ages of 35 to 45, when there are no other identifiable causes. (health.com)