Final answer:
Menarche generally occurs around 12 or 13 years of age but can vary from 8 to 16. Factors influencing this include genetics, nutrition, body fat levels, and environmental conditions. The average age of menarche in the U.S. has decreased over time due to improvements in nutrition and health.
Step-by-step explanation:
The onset of menstruation, also known as menarche, usually occurs when girls reach puberty. Menarche signifies the beginning of a woman's reproductive years and typically happens around the age of 12 or 13, but it can vary from about 8 to 16 years of age. This variation in age can be influenced by several factors including genetics, nutrition, body fat, and environmental factors.
In the United States, the average age for menarche has changed over time, with historical data showing that it has decreased from about 17 years in the 1860s to around 12.75 years by 1960. Factors such as improved nutrition and health care have contributed to this change. It is also noted that body fat and the hormone leptin play a role in the timing of menarche, as girls who are lean and highly active may experience a delay in the onset of puberty.