12 and 1/2. For african americans about age 12 and a few months. For whites about 12 and a half. There have been at least five different studies since 2000 which gave slightly different results.
12.5. The average age for onset of menstrual cycles is 12.5 the normal range is ages 9-15.
Historically 12.5. But with the obesity epidemic it has moved up a couple years as has secondary sexual devleopment. The reason being - usually the body has to have a certain percentage body fat to begin menarche, thus athletic girls tend to have delayed menarche. A 2011 study found that each 1 kg/m2 increase body-mass index (bmi) can be expected to result in a 6.5% higher absolute risk of early menarche.
Dropping. The age at which a girl has the first period depend in several factors. Proper thyroid function and a certain ratio of body fat are necessary. With the increase in calories in modern society, this age is ever younger. In the U.S. , for african american girls closer to 8.5 years old . For other ethnic groups it's closer to 9. Read more...