Facial hair is often present in late adolescence, around ages 17 and 18, but may not appear until significantly later.
Consequently, girls who have a high-fat diet and who are not physically active begin menstruating earlier, on average, than girls whose diet contains less fat and whose activities involve fat reducing exercise (e.g. They have the advantage in capturing the attention of potential partners and in becoming hand-picked for sports.
The end of adolescence and the beginning of adulthood varies by country and by function.
Furthermore, even within a single nation state or culture there can be different ages at which an individual is considered (chronologically and legally) mature enough for society to entrust them with certain privileges and responsibilities.
This is triggered by the pituitary gland, which secretes a surge of hormonal agents into the blood stream, initiating a chain reaction to occur.