Final answer:
Menarche is the first menstrual period in females, marking the start of menstruation, with the average onset age in the U.S. being 12.4 to 12.5 years. Variations in this age are normal and can be influenced by genetics, environment, and nutrition.
Step-by-step explanation:
Menarche is a significant developmental milestone in female puberty, signifying the first menstrual period. This event marks the commencement of menstruation, a regular biological process involving the shedding of the uterus lining. In the United States, the average age for menarche is approximately 12.4 to 12.5 years, but it can normally occur anywhere between the ages of 8 and 16. Factors contributing to the age at which menarche occurs include genetics, environment, nutrition, and body composition such as the amount of stored fat. Improved nutrition has been associated with a historical decrease in the age of menarche, which underscores the link between environmental factors and the onset of puberty.