Menstruation
Menstruation is the most visible phase of the menstrual cycle and its beginning is used as the marker between cycles. The first day of menstrual bleeding is the date used for the last menstrual period (LMP). The typical length of time between the first day of one period and the first day of the next is 21 to 45 days in young women, and 21 to 31 days in adults (an average of 28 days).
Woman
Menarche, the onset of menstruation, occurs on average at age 12–13. Many cultures have rites of passage to symbolize a girl's coming of age, such as confirmation in some branches of Christianity, bat mitzvah in Judaism, or even just the custom of a special celebration for a certain birthday (generally between 12 and 21), like the quinceañera of Latin America.