Two of the leading causes of early puberty, or "precocious puberty," include hormone-disrupting chemicals and vitamin D deficiency. http://articles.mercola.com/sites/articles/archive/2012/04/16/early-precocious-puberty.aspx
A study about precocious puberty shows that environmental exposure to endocrine-disrupting chemicals plays a major if not decisive role. http://articles.mercola.com/sites/articles/archive/2015/12/30/precocious-puberty-endocrine-disrupting-chemicals.aspx
magic foundation for children's growth : Central Precocious Puberty. Precocious Puberty means having signs of puberty (e.g., pubic hair or breast/ testicular enlargement) at an earlier age (before age 8 in girls and age 9 in boys). Normal Puberty - Girls usually develop breasts and then pubic hair between the ages of 8 and 13 years. Boys normally develop testicular enlargement and then pubic hair between the ages of 9 and 14 years.