Puberty starts on average in girls between ages 8 and13 and in boys between ages 9 and 14. Doctors diagnose early puberty when this normal process starts early and continues to progress through growth spurts and bone maturation, usually for reasons we don't understand. These hormones in turn stimulate the testicles or ovaries to make other hormones, testosterone or estrogen. 2  It's these sex hormones that cause the changes of puberty, like breast development in girls. 3  Peripheral precocious puberty or precocious pseudo-puberty is a different condition. 4  It's also rarer.