When are American girls hitting puberty ?

12.3 years old -- this is the average age of onset for American girls hitting puberty in 2000s. American girls are hitting puberty earlier than ever. According to December 2005 issue of Journal of Pediatrics, there is a gradual decline in the average age at which U.S. girls have their first menstrual period - from the age of 12.75 in the 1960s to about 12.5 in the early 1990s. One reason: girls are getting fatter, which triggers menstruation.