may be physically mature and a biological adult by age 16 or so, but not defined as an adult by law until older ages. For example, in the US, one cannot join the armed forces or vote until age 18, and one cannot take on many legal and financial responsibilities until age 21.

Human adulthood encompasses psychological adult development. Definitions of adulthood are often inconsistent and contradictory; a person may be biologically an adult, and have adult behavior, but still be treated as a child