the state (and responsibilities) of a person who has attained maturity
"Taking on full responsibility for his own decisions marked the true beginning of adulthood for him."
The state or condition of a human being once it has reached physical maturity, and is presumed to have reached a state of psychological maturity, to wit: once it has become an adult.
"After years of feeling lost in his early twenties, he finally embraced the responsibilities that come with adulthood."