Please read the article again. Menstruation is not the "first sign". There are other signs before menstruation.....like small breast buds. And while I think the mentioned six or seven is young, in my experience with my four daughters and other girls I've been around, it's not that far off the mark, unfortunately.
Of course, if the average is 13, then then 1/2 of the girls will be younger than that. I am sure that there are plenty of girls developing at 10,11,12. The question is, has the average age dropped. I would love to see a large study on the subject.