1. the period from the late teens to the mid-late 20s, when individuals aren't adolescents but aren't fully adults A time of exploration of careers, identity, commitments; marks the peak of many biological and physiological developments Poses risks to young adults because they may have unrealistic expectations of adult life, and behave in riskier ways. The most widely used criteria for deciding whether a person has reached adulthood, which involve assuming the new responsibilities and duties of adults.