In my family, each generation of women has begun menstruation about one year earlier than their mother: my Mum was 13, I was 12, my DDs were both 11. We had talked about it with them openly whenever they asked about my personal 'products' in the bathroom - with the aim that they would not experience any anxiety when it was their turn. Also, from when my DDs were 10 or so, I kept a variety of products in the bathroom just for them to remove any embarassment or confusion they may have experienced.
I do feel for those children who are much younger - life can be tough sometimes on the young cant it.