http://articles.mercola.com/sites/articles/archive/2002/10/23/period.aspxThe average age of a girl's first period is around 12. The research included data from over 7,500 girls. Researchers are interested in the reason for the difference in the onset of menstruation because women who get breast cancer are more likely to haveenCommunityServer 2008.5 SP1 (Build: 31106.3070)