Menstruation is usually normal between the ages of 11 and 15, and the age of menarche is usually between 13 and 14, but it can be as early as 11 or as late as 15. If menstruation does not occur after the age of 15, it should be a clinical concern. The early or late onset of menstruation is mainly influenced by genetic factors, but other factors, such as nutrition and weight, also play an important role. The first menstrual flow in women is called menarche, which is an important sign of puberty and is on average 2.5 years after breast development. The onset of menstruation indicates that the ovaries produce enough estrogen to proliferate the endometrium, and when estrogen reaches a certain level and fluctuates significantly, it can cause the endometrium to shed, i.e., menstruation occurs. At this time, the central positive feedback mechanism to estrogen is not yet mature, so menstruation is often irregular. It usually takes 5-7 years to establish regular cyclic ovulation before menstruation gradually becomes normal.