Uneven breast growth and soreness are both totally normal and usually be the first sign of puberty―showing up before breast budding starts.
Thelarche is the onset of secondary breast development, which often represents the beginning Thelarche, also known as breast budding, typically occurs between the ages of 8 and 13 years with significant variation between individuals .
breast budding, early. The day may come when your preadolescent girl (perhaps eight or nine years old) confides that her breast or breasts have lumps right.
Breast development is a big part of getting older for girls. For help, advice and information on budding boobs, check out Becoming a Teen.