Final answer:
Education is not commonly used as a sole criterion for defining adulthood; whereas age, financial independence, and marriage are traditional markers. Adulthood consists of a collection of societal milestones, which can vary greatly and are shifting over time, especially among Millennials.
Step-by-step explanation:
Of the criteria listed for defining adulthood, the one that is NOT most commonly used is education. While it is true that completing school is often seen as a transitional stage into adulthood, it is not typically a criterion on its own for defining whether someone is an adult. In comparison, reaching a certain age, achieving financial independence, and entering into marriage are traditional markers that society tends to associate more directly with the adult status.
Adulthood is complex to define due to variations in societal expectations, legal definitions, and individual development. Unlike other life stages, it is not marked by a single event, but rather a collection of milestones that can include leaving home, becoming financially independent, getting married, and having children. These markers may differ culturally and have shifted over time, particularly for the Millennial generation, which exhibits delays in reaching these milestones, largely due to economic and cultural factors.