Reference: Fataawa al-Lajnah ad-Daa.imah lil-Buhooth al-‘Ilmiyyah wal-Iftaa. – Fatwa No.1790
Fataawa Ramadhaan – Volume 2, Page 583, Fatwa No.567
Question: An old lady of sixty years of age was ignorant concerning the rulings of menstruation for many years. Throughout her menstrual years, she did not make up the days missed in Ramadhaan thinking they did not have to be made up. This was according to what she had heard from the sayings of the people in general.
Response: She has to seek repentance from Allaah (Subhaanahu wa-Ta’aala) for that [which has passed] because she had not asked the people of knowledge. Also, she has to make up all the missed days and make expiation for each day missed by feeding a poor person half a measure of barley, dates, rice or that which is similar to this from the food of people. This is if she is able to do so. If she is unable to feed the poor, then this is (no longer) required and it will suffice that she make up the days missed only.
And with Allaah lies all the success, and may Allaah send prayers and salutations upon our Prophet Muhammad (sal-Allaahu ‘alayhi wa sallam) and his family and his companions.