Hello everyone,
  I have not followed the whole discussion on age of menarche so this
may already ahve been mentioned but ... I remember reading an article
in a popular journal and few years ago (which one escapes me at themoment)
which referred to some research that had been done on rats I believe.
The research suggested that age of onset of menarch may be related, at
least in part, to breastfeeding. That is that something in breastfeeding
delays the onset of menstruation. I wish I could remember the details,
but it was a fascinating study and I wonder how it could be verified
and/or correlated to humans. ONe would need a large smaple of mothers
and daughters who nursed for a long time and who had adequate nutrition etc.
  I also enjoyed the article in U.S. News and World Report. The author
interviewed me, having heard that I wrote my dissertation on breastfeeding
and the workplace. We had a long and detailed conversation but you
never know how these things will make their way into print. I was
pleased that she did her research so well and that the article was so
well done. I only hope that it sparks some companies to take steps to
support breastfeeding (dare I hope that some companies actually
promote it!?)
  Naomi Bar-Yam PhD