Padded bras for kindergarteners  with growing breasts to make them more comfortable? Sixteen percent of U.S. girls experiencing breast development by the age  of 7? Thirty percent by the age of 8? Clearly something is affecting the hormones of U.S. girls—a phenomenon also seen in other developed countries. Girls in poorer countries seem to be spared—until they move  to developed countries.
No scientists dispute that precocious or early-onset puberty is on the rise but they do not agree on the reasons. Is it bad diets and lack of exercise that cause growing obesity? Is it soft drinks  themselves, even when not linked to obesity? Is it the common  chemicals 
Puberty in girls is defined by three  things: breast development (thelarche), appearance of pubic hair (pubarche) and the onset of menstrual periods (menarche), the latter coming last. In the 1700s, girls did not menstruate until  age 17 or 18, and 100 years ago the average age when a girl got her first period was  16-17.