Isolated breast development that doesn’t progress to the rest of puberty is called premature thelarche, and is a different, benign condition.
Precocious puberty is 10 times more common in girls than in boys. Sexual development may begin at any age. Pregnancy has been reported as early as 5 1/2 years old.
The Lawson Wilkins Pediatric Endocrine Society recommends evaluating for an underlying medical condition in Caucasian-American girls who have development of breast and/or pubic hair before age seven and in African-American girls before age six (Kaplowitz and Oberfield, Pediatrics 1999 Oct;104(4 Pt 1):936-41). These medical conditions include tumors, cysts, thyroid problems, McCune-Albright syndrome, or external sources of estrogen. Doing studies to look into these possible causes is especially important in girls younger than 6, and in all boys.
The earlier before age 12 a girl starts her period, the higher her lifetime risk for breast cancer (probably from the prolonged estrogen exposure). The highest average risk for breast cancer is in non-Hispanic white women, where it is 1 in 8, or 12.5%. In all girls who start their periods before the age of 12, taken together, the risk is 16.25%.
As a girl reaches maturity, she needs to be made aware of controllable risk factors for breast cancer, such as use of estrogen-containing birth control pills (10 years of use would raise her risk to about 22%), first pregnancy after age 30 (if she did this also, it would raise the risk to about 35%), high-fat diet, alcohol use, fertility drugs, pesticides, and radiation exposure. Each of these factors multiplies her accumulated risk.