Final answer:
Puberty in girls in the USA typically begins between the ages of 9 and 10, with menarche occurring at an average age of 12.4 years, but can vary widely. Nutritional improvements have contributed to an earlier onset of puberty over time.
Step-by-step explanation:
In the USA, puberty begins at ages 9 to 10 for most girls. The onset of puberty is marked by visible changes such as the development of breast tissue and pubic hair, followed by a growth spurt between ages 9 to 11. A significant milestone in female puberty is menarche, signifying the start of menstruation, which on average occurs at 12.4 years but can range between 8 to 16 years and is considered normal.
These biological changes during puberty are influenced by a combination of factors including genetics, nutrition, environmental influences, and body composition. Improvements in nutrition, for example, have led to an earlier onset of puberty over time, with historical data showing a decrease in the average age of menarche in the United States from around 17 years of age in the 1800s to approximately 12.75 years in the 1960s, a trend that remains today.