The onset of puberty in girls is characterized by the development of certain female secondary sexual characteristics which are controlled by hormones one of which is estrogen. Increase in breast cancer incidence in woman has been associated with a higher exposure to the hormone estrogen. While most girls in the past got their period between ages 14-17, girls in recent times now get their periods between ages 8-12 and the longer a woman menstruates, the longer her exposure to this hormone.

Breast cancer has proven to be one of the deadliest diseases of the 21st century killing more people than every other cases of cancer in women. According to the National Cancer Institute, 232,340 female breast cancers are reported each year in USA accounting for 16% of all female cancers and 22.9% of invasive cancers (invasive cancers are not easily treated and they spread rapidly) in women. Compare two women, who got their menarche (first menstruation) at ages 11 and 15 and reached menopause at ages 55 and 45 respectively. The latter would have had about 30years of exposure to estrogen and former having about 44years of exposure increasing her chances of developing a breast cancer.

The sudden shift and decline in the age at menarche is attributed to obesity (been overweight), increase in hormonal disrupter present in some products. Having more fats in your body is like telling your body that you are ready to carry a baby since puberty requires a great deal of energy even though you are not- psychologically. Girls who are lean and exercise regularly tend to get their period later in their adolescence.
A woman might not be able to influence the age she got her first period but a girl can. Parents should engage their daughters in more activities and exercises to get rid of excess fats. Girls should be encouraged to spend more time outside than being indoors-in front of television screens and smartphones. As a woman, adopting a healthy lifestyle and good nutrition will help in lowering your chances of developing a breast cancer.

https://ww5.komen.org/BreastCancer/FirstPeriodOccursBeforeAge12.html