The development of a 9-year-old girl is normal. Normal development for girls can start after the age of 8, but the exact time of onset of development is determined by genetics, dietary habits and medications. Development can start as early as 8 years old, when the ovaries develop and secrete certain sex hormones, resulting in the accumulation of subcutaneous fat and the development of breasts, which begin to show female characteristics. As a girl grows older and enters puberty (10-19 years old), her reproductive organs and body gradually develop into a mature stage. Early puberty in girls, also known as precocious puberty, is when a girl starts to develop before the age of eight. Causes of precocious puberty include early development of parents, disease factors such as ovarian tumors, dietary factors such as consumption of junk food and misuse of contraceptive pills. If you see signs of precocious puberty in your child, you should seek medical attention promptly.