The inside of the breast is made up of fatty tissue and milk-producing glands called mammary glands. Breast development begins with puberty and is caused by the changes in hormone levels in your body. The timing of breast growth is determined by heredity and nutrition. On the average, breasts begin to grow when girls are around 10 years old, although some girls start earlier and others later. The age at which breast development begins does not affect the final size of the breast. Breast shape and size are determined mostly by heredity and usually stay the same once development is complete (which takes 3-5 years). However, excessive weight loss or gain, and pregnancy, may cause changes to the breasts. A girl may find her breasts to be uneven in size during development. This is normal. The breasts will be about the same size as the girl gets older, usually by age 20. You can check with your health care providers for further information.