According to the dictionary, In the human context, the term adult refers to social and legal notions. A legal adult, as opposed to a "minor," is a person who has reached the age of majority and is thus considered independent, self-sufficient, and accountable.
When a person reaches the age of maturity, they are considered an adult. In the state of California, a person is considered an adult when they reach the age of 18, but not everyone who reaches that age is mature or competent to make sensible decisions.
However, considering a person an adult at the age of 21 is logical because many people at that age are more likely to be mature. At the age of 21, a person should be regarded as an adult since they are wiser, have less peer pressure, and are more responsible.