When you stop growing and start shaving, you are in adulthood — a point where you reach maturity. Unfortunately, physical adulthood does not necessarily imply that psychological adulthood is present.
Adulthood implies a maturity of sorts, whether it be physical or psychological. In many cultures, adulthood is marked with a “coming of age” ritual. In Judaism for example, adulthood is marked by the Bat Mitzvah for girls (age 12) and the Bar Mitzvah for boys (age 13) — in which the girl or boy is considered to have reached adulthood within the community.