Most schools start educating girls on menstruation starting in the 5th grade. Because many girls begin to menstruate in the 6th grade (some in 7th), so they're prepared before hand.
I teach, so I'm basing it on that. When I used to teach 4th grade, a few of my students were already aware of it.
So......I would say 5th grade (age 11) would be a good time to start. Every girl develops differently, and there are actually girls who begin to menstruate irregularly and even regularly as early as 11.
I've read that a good strategy parents use when discussing awkward subjects is to ASK your child what they ALREADY know about the topic (or have heard about the topic)..........BEFORE discussing it. Because kids get their information from other kids and even the media, so sometimes they already have a little general knowledge about the subject. This can break the ice a bit and help make the situation a little less uncomfortable. And also, ask if they have any questions or concerns during/after the discussion.
You can refer to books to get ideas. The school nurse/PE teacher usually explains the reason for menstruation, characteristics, symptoms, what to use (napkins/tampons), overall changes in body development, implications of menstruation (irregular periods, pregnancy, safe sex) etc. As a parent, of course, you would use your judgment as to how you should explain menstruation, for example incorporating Islamic guidelines and rules.
Over all, young girls tend to handle the discussion aspect well. Actually dealing with the monthly visits from Aunt Flow are far more annoying than any discussion mommy or the nurse or the pe teacher can conduct, LOL:)