Making Up Missed Fasts from Years Ago
Question

There is a woman who has reached the age of fifty. When she got her period in Ramadan she did not fast those days, but then she did not make them up because she was unaware that it is obligatory to make up missed fasts. Now she has come to know that it is obligatory to make up for missed fasts. What should she do?
Praise be to Allah.She has to make up for those days, and to be on the safe side she should feed one poor person for each day. 
Shaykh Ibn Baz (may Allah have mercy on him) was asked (15/184): 
I have a sister who for several years did not make up the fasts that she missed because of her period, because she was unaware of the ruling, especially since some of the common folk told her that she did not have to make up the fasts that she missed. What should she do? 
He (may Allah have mercy on him) replied: 
“She has to ask Allah for forgiveness and repent to Him, and she has to fast the days that she missed and feed one poor person for each day , as was stated by a number of the companions of the Prophet (peace and blessings of Allah be upon him). This means giving half a Sa`, which is equivalent to one and a half kilograms. What some ignorant women told her about her not having to do anything does not mean that this obligation is waived. `Aishah (may Allah be pleased with her) said: “We were commanded to make up the missed fasts but we were not told to make up the prayers.” (Narrated by Al-Bukhari and Muslim) 
If another Ramadan comes before she makes up the missed fasts, then she is sinning and she has to make up the missed fasts and repent, and also feed one poor person for each day if she is able to.
If she is poor and cannot feed others, then fasting and repentance is sufficient and she does not have to feed others. 
If she cannot remember how many days she has to fast, she should proceed on the basis of what she thinks is most likely, and fast the days that she thinks she missed during Ramadan, and that will be sufficient, because Allah says (interpretation of the meaning): 
“So keep your duty to Allah and fear Him as much as you can.” [At-Taghabun 64:16]”
The Standing Committee (10/151) was asked about an elderly woman who had reached the age of sixty, and she was unaware of the ruling on menstruation for all the years during which she used to menstruate, so she did not make up any missed Ramadan fasts because she thought that she did not have to make them up, based on what she had heard from the common folk. 
The committee replied: 
“She has to repent to Allah from that, because she did not ask the people of knowledge. As well as that she also has to make up the days that she did not fast, according to what she thinks is most likely to be the number of days that she missed, and she should offer expiation for each day by feeding one poor person half a Sa` of wheat, dates, rice or whatever is the staple food of the country, if she is able to do so. If she cannot feed others, then she does not have to do that and it is sufficient for her to make up the fasts.” 
For more details, please see the following answers: 21697 , 192428 , and 95736 .
And Allah knows best.