Breast development in nine year olds has no effect on height and is a normal part of development. Girls can start to develop as early as after the age of eight, but the timing of development varies from person to person. As girls grow older, the ovaries begin to develop and secrete sex hormones after the age of 8 at the earliest. Subcutaneous fat begins to accumulate, and breasts begin to develop and show feminine characteristics. Generally, female breasts develop near the age of 10, but some girls may start to develop at the age of 8 to 9 due to good nutrition, obesity, and hereditary factors such as early development of their parents. So the development of 9-year-old girls is normal development, will not affect the child’s height. If parents want their children to grow taller in the supplement enough nutrition at the same time, take the child more exercise and keep enough sleep.