Modern foods have a LOT to do with the pain and cramping! Who's to say about the other "PMS symptoms"...
I believe the book "Sugar Blues" explained that only a couple hundred years ago only the elite could afford the high-cost sugary foods and processed flours; they were also the only ones (back then) who had a LOT of the maladies today's regular folk have - due now to the inexpensive (dollar-wise, anyway) availability of those foods!
Within that chapter, it was explained that the menstrual cycle (of those who ate off of the land, and not those raised by kings and queens) typically started when a girl was older than today's starters - and - they did NOT have pain and cramping or awful mood swings....