Adults are biologically identified as organisms that can reproduce. For human anatomy, that could be measured by menstruation in females and ejaculation in males; they must have, in the least, have started going through puberty.
However, adulthood could also mean being capable of caring for offspring, which is consequently much harder than having sexual intercourse.
All of this said, don't show this to a cop after being caught underage in the bar drinking beer. Most countries' LEGAL adulthoods begin around 17-21 years of age, and that for various reasons.