The age at which a girl’s mammary glands develop is not generalizable; the average age is 10 to 11, and most are symptomatic. The mammary glands usually begin to develop when a girl is about 10 years old, usually before the onset of menstruation. When the breasts first begin to develop, the nipples are raised, the breasts and nipples are mildly elevated, and the areola is enlarged. With gradual growth and enlargement, the breasts and areola further increase in size and projection, and a secondary mound-like elevation develops on the breasts. When the mammary gland is fully developed the areola and breast return to the same mound. There are individual differences in the time and size of mammary gland development. If a girl’s mammary glands develop before the age of 8, she should be alerted to the possibility of precocious puberty. If a girl does not develop by the age of 16, she needs to be alerted to the possibility of delayed sexual development and ovarian dysplasia. The symptoms of mammary gland development in girls are more obvious, if the development is too early or delayed, you should take your child to the hospital in a timely manner, to clarify the cause of the disease and treat it as soon as possible.