“Adulthood is the age at which a person learns [they] must die…and accepts [their] sentence undismayed.”
-R. Heinlein in The Moon is a Harsh Mistress
A friend suggested: adulthood is being able to comprehend the impact of our actions on the larger whole, accepting the responsibility of this knowledge, and choosing to act accordingly.
I feel these are two aspects of one truth. There, I am sure, are many more ways to expound.