The normal developmental age of girls usually starts from 11-12 years old, and most girls will stop developing when they are 17-18 years old. Of course, due to individual differences, some girls may start to develop earlier, usually around the age of 10, while some girls may develop later and need to wait until the age of 13-14 before they start to develop their second sex characteristics, which are normal. Individual differences are influenced by many factors, such as genetic factors, ethnicity, geographical factors, diet and nutrition, and disease. When girls reach puberty, secondary sexual characteristics begin to appear, starting with the development of breasts, with larger nipples and enlarged areolas, as well as an upward bulge. Pubic hair and armpit hair will also begin to grow, while height and weight will enter a stage of rapid growth, and then the first menstruation will appear as the age increases. In addition, if a girl develops secondary sexual characteristics before the age of 8, precocious puberty may be considered in this case. There are two main causes of precocious puberty: on the one hand, it is considered to be central precocious puberty, commonly caused by abnormal development of the central pituitary gland or early activation of the hypothalamic-pituitary-gonadal axis; on the other hand, it is considered to be peripheral precocious puberty, which may be related to hormone-containing foods in the diet. If a girl is more than 15 years old and still has not reached puberty, she may have delayed puberty and needs to go to the hospital in time to find out the cause so that targeted treatment can be given.