"The `Iddah of Those in Menopause and Those Who do not have Menses Allah the Exalted clarifies the waiting period of the woman in menopause. And that is the one whose menstruation has stopped due to her older age. Her `Iddah is three months instead of the three monthly cycles for those who menstruate, which is based upon the Ayah in (Surat) Al-Baqarah. [see 2:228] The same for the young, who have not reached the years of menstruation. Their `Iddah is three months like those in menopause. This is the meaning of His saying. ..."
January 1, 1970
https://en.wikiquote.org/wiki/Divorce_in_Islam