Adulthood is generally understood as the time when physical maturation is complete. Some examples of physical maturation include when an individual reaches their maximum height and secondary sex characteristics form such as body hair and facial hair, voice lowers in pitch (especially noticeable in men), and menses begin (women).

Natural sleep patterns change in adulthood, as adults typically require less sleep than during adolescence.

One aspect people do not specify is what (psychologically) an adult is. A common theory is that adulthood is the real test of life, to experience the world from a first-person standpoint instead of through the parents. Then the adult can pass those experiences down to younger people and they can experience them when they become adults.