The term “period” generally refers to menstruation. The age at which a girl has her first menstrual period is the age of menarche. Generally speaking, most people have their first menstrual period between the ages of 13 and 14 years old, while a small number of people may have it as early as 11 years old or as late as 16 years old. When girls are 8 to 10 years old, the negative feedback mechanism of the central nervous system is lifted and the ovaries begin to secrete sex hormones, and the uterus under the action of the sex hormones exhibits cyclical shedding of the lining and bleeding, which is the onset of menstruation. At the beginning, due to the imperfect ovarian function, the menstrual cycle is irregular, with the further improvement of ovarian function, menstruation begins to be regular. Due to genetic, nutritional and environmental factors, there are individual differences in menstruation. In recent years, there is a tendency for the age of menarche to be earlier, so if the age of menarche is younger than 11 or 16 and you still do not have menstruation, you need to pay attention to it and go to the hospital for checkups and related treatments.