Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.069.

In Arabic, the word for menstruation (haizh) literally means “running.” Here it refers to the discharge of blood during a woman’s state of health, not from giving birth or breaking the hymen.

Most scholars say that its time begins at the age of nine. If blood is seen before that age, it is not menstrual blood, but is considered to be putrid blood. As there is no evidence about when a woman stops menstruating, if an elderly lady finds blood flowing it is considered menstrual blood.