To understand why women who begin menstruating at a young age are at a slightly elevated risk of developing breast cancer, one must understand the role of estrogen. Estrogen alone does not cause cancer. The hormone, estrogen, is an essential part of a womanís physiology.
However, estrogen's principal function is to speed the process of cell proliferation. Therefore, estrogen can increase the chance of a mutation occurring and/or encourage the growth of cancerous cells once they appear.
The more estrogen a woman is exposed to during her lifetime, the greater the opportunity for the hormone to promote the growth of a tumor. The reason why women who began menstruation at an early age are at greater risk of developing breast cancer is because these women have had a longer lifetime exposure to estrogen.