I read an article in Scientific American a few years ago that discussed
the relationship of age at menarche with the ratio of lean to fat body
mass.  It seems a girl has to reach a certain proportion of fat before
the menstrual cycle can begin.  This is why athletes and others with low
percentage of body fat begin at a later age or maybe not at all while
they are exercising so much.  It may also explain why higher nutritional
status results in earlier menarche.  (I was never very athletic and began
at age 11.  My daughter, an enthusiastic gymnast, didn't begin til 14.)
I wish I could remember the month and year of that issue, but we had
checked it out of the library.

Kris Rogers, RN, BSN,IBCLC  Crystal Lake ,Il