How does a teenager become an adult?
Is it something intrinsic? Like, does a person become an adult because of the way they think of themselves? Is it when they except responsibility for themselves internally and start making adult-like decisions? Is it putting them on a pathway towards independence? (Vocation, education, relationships)
Or is it extrinsic? Do you cross a threshold physically to become an adult? Does turning 18 years old make you an adult? Does achieving some physical characteristic make you an adult? Does some level of educational achievement or military service make you an adult?