Girls experience menarche at different ages. Having menarche occur between the ages of 9–16 is considered normal. The timing of menarche is influenced by female biology, as well as genetic and environmental factors, especially nutritional factors. The mean age of menarche has declined over the last century, but the magnitude of the decline and the factors responsible remain subjects of contention. The worldwide average age of menarche is very difficult to estimate accurately, and it varies significantly by geographical region, race, ethnicity and other characteristics. Various estimates have placed it at 13. There is a later age of onset in Asian populations compared to the West. The average age of menarche is about 12.5 years in the United States, 12.7 in Canada, and 12.9 in the United Kingdom. A study of girls in Istanbul, Turkey, found the median age at menarche to be 12.7 years. 
Girls experience menarche at different ages. Having menarche occur between the ages of 9–16 is considered normal. The timing of menarche is influenced by female biology, as well as genetic and environmental factors, especially nutritional factors. The mean age of menarche has declined over the last century, but the magnitude of the decline and the factors responsible remain subjects of contention. The worldwide average age of menarche is very difficult to estimate accurately, and it varies significantly by geographical region, race, ethnicity and other characteristics. Various estimates have placed it at 13. There is a later age of onset in Asian populations compared to the West. The average age of menarche is about 12.7 years in Canada, and 12.9 in the United Kingdom. A study of girls in Istanbul, Turkey, found the median age at menarche to be 12.7 years. In the United States, an analysis of 10,590 women aged 15-44 taken from the 2013-2017 round of the CDC’s National Survey of Family Growth found a median age of 11.9 years (down from 12.1 in 1995), with a mean of 12.5 years (down from 12.6). URL
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In the United States, an analysis of 10,590 women aged 15-44 taken from the 2013-2017 round of the CDC’s National Survey of Family Growth found a median age of 11.9 years (down from 12.1 in 1995), with a mean of 12.5 years (down from 12.6). URL