The stages of breast development are generally judged according to the characteristics of breast development, such as nipple diameter, areola size, breast morphology, and are mainly divided into five stages, which cannot be distinguished by age due to large individual differences. 1. The first stage: generally between the ages of 1 and 9, the mammary glands are not developed, there is not much change, the nipple is generally below 3mm, and the areola is flat. 2. Stage 2: mainly occurs around 10-11 years old, nipples and areolas begin to develop, and a raised hard lump can be touched. 3. Stage III: At around 12-13 years old, the breasts grow rapidly, swell, and gradually form a hemispherical shape, and the nipple reaches an average value of 4mm. 4. Stage 4: Usually around the age of 14-15 years, the nipple and areola further increase in size and continue to bulge, forming a nipple that shows a clear demarcation line from the breast, while the underbelly of the breast continues to develop. 5. Stage 5: Mainly at the age of 16-18 years, the nipple, areola and overall breast gradually fuse and develop to form a mature breast. The development of breasts is related to many factors and there are individual differences. If there is premature or delayed development, you should go to the hospital for diagnosis in time to avoid delaying the condition.