More and more girls are starting menstruation between age 9 and age 12[1] - medical research suggests it's partly a high fat and/or high sugar diet; partly hormones in food; and also girls without an involved father start menstruation early[2].

So explaining about menstruation should happen by age 8 or age 9.

[1] Early puberty in girls doubles in a decade
http://www.newscientist.com/article/dn19288-early-puberty-in-girls-doubles-in-a-decade.html

[2] Father-Daughter Relationship Crucial To When Girls Enter Puberty, Researchers Say
https://www.sciencedaily.com/releases/1999/09/990927064822.htm