Final answer:
The correct statement about menarche is that 1) the average age is earlier for Black, non-Hispanic girls compared to White or Hispanic girls. It is influenced by genetics, nutrition, environment, and body fat levels, with improved nutrition contributing to an earlier age of onset over time.
Step-by-step explanation:
The true statement about menarche is that the average age of menarche is earlier for Black, non-Hispanic girls than White or Hispanic girls. Menarche is influenced by several factors, including genetics, environment, and nutrition. Nutrition and body fat, in particular, as indicated by the secretion of the hormone leptin by adipose cells, play a significant role in the timing of menarche. For example, girls who are lean and highly active, like gymnasts, might experience a delay in the onset of puberty. It is also worth noting that the age of menarche has decreased over time, moving from an average age of approximately 17 years in 1860 to around 12.75 years in 1960, a trend that has remained stable up to the present day.