Reference: al-Fataawa as-Sa’diyyah – Page 103, Question 1
Question: If a woman has reached 70 years of age, and her [menstrual] blood continues [upon its cycle], should she assume it is still her menses [at such an elderly age]?
Response: A woman who has reached 70 years of age and her [menstrual] blood continues [upon its cycle] should not disclaim it, rather she should assume her menstrual state. This is because there is no lower age limit for the start of the menstrual cycle and nor is there an upper age limit for it ceasing. So the ruling for the blood [which continues to flow] is that of the ruling of menses from all aspects.