The historical decline in the average age of menarche in the United States, from about 17 years in 1860, has stabilized at approximately what age since 1960?

1. Nutrition is one important factor influencing the timing of puberty onset. 2. Historical data track how average menarche age changed with improving nutrition in the United States. 3. The 1860 average as about 17 years, declining over a century. 4. 9 years and 15 years are both different figures a reader might guess from memory, but neither matches the specific figure given here. 5. 18 years is also incorrect for the same reason: it does not match the specific figure given here. 6. It states the age stabilized at approximately 12.75 years by 1960, and it remains close to that figure today. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 27 "The Reproductive System", p.1311 §Further Sexual Development Occurs at Puberty_