Nine-year-old girls breast development should pay attention to a reasonable diet, avoid breast binding, appropriate exercise and so on. 1. Reasonable diet: you can properly supplement protein and vitamins, such as lean meat, soy products, etc. Do not diet and lose weight because of breast growth, so as not to affect the normal development of the mammary glands. 2. Avoid breast binding: you can choose suitable underwear, breast binding may lead to nipple inversion, which may affect future breastfeeding, and may even lead to nipple infection, suitable underwear can reduce the swinging of the breasts and alleviate the pain caused by the development. 3. Appropriate exercise: You can do appropriate chest exercises, and sports that match your physical strength to avoid injuries to your breasts. If a girl starts to develop at the age of 9, she should pay proper attention to it, and if there are any abnormalities such as nodules, she can consult a doctor.