When my mother was about 14 or 15 years old, she witnessed her elder sister giving birth to her child. She panicked and said, "If my sister survives this delivery, I will fast four successive months." She was ignorant of many sharia rulings at that age. She hardly knew how to pray and recite the Fatihah (1st chapter of the Quran). My mother belongs to the wandering nomads, and she does not know whether she was competent for religious assignment at that time or not. Is she obliged to fulfill this vow, given that she is now over 45 years old?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His Slave and Messenger.

The opinion that we adopt is that your mother was most probably competent for religious assignment when she took that vow, even if she does not remember that she had any of the signs of attaining puberty, such as: having wet dreams, growth of coarse pubic hair around the sexual organ, or experiencing menstruation. Hence, we would judge that she has to fulfill her vow because she was competent for religious assignment when she had made it. This is because women at that age are most probably competent for religious assignments. They may even attain puberty at the age of nine or ten. Therefore, we rule out the possibility that she might have been incompetent for religious assignments at that age. It is incumbent on her to fulfill her vow in the exact manner that she had vowed as long as she can do so, as it is more likely that she had attained puberty at that time. It is well-known that the religious ruling is based on matters that are mostly certain. Allaah, the Exalted, also says (what means): {…and fulfill their vows …} [Quran 22:29]

 The Prophet, sallallaahu ‘alayhi wa sallam, said: "Whoever vowed to offer an act of obedience to Allaah, the Exalted, must do..." [Al-Bukhari and others]

What we understood from the information in your question is that she was aware of vow-taking and of observing consecutive and intermittent fasting. If this is the case, she will not be excused for ignorance, and she is obliged to fulfill her vow, as we underlined, unless she is totally unable to fulfill it in a way that is not likely to end. In such a case, she may offer an expiation like that for breaking an oath. The Prophet, sallallaahu ‘alayhi wa sallam, said: "Whoever makes a vow and is not able to fulfill it, the expiation for it is like the expiation for breaking an oath…" [Abu Daawood and Ibn Maajah] The version of Ibn Maajah reads, "... Whoever makes a vow and is able to fulfill it, let him do so." [Ibn Maajah]

If your mother was raised in an ignorant environment and was unaware of the meaning of her words when she uttered the vow, then she is to be excused for ignorance, and it is not incumbent on her to do anything in this regard. There are religious texts that indicate that ignorance is a Sharee'ah-acceptable excuse.

The Maaliki scholar, An-Nafraawi stated:

"The person's ignorance of the meaning of his words waives the due punishment. It was reported on the authority of Al-‘Izz ibn ‘Abdu As-Salam that the Muslim who is competent for religious assignment will not be held accountable for words if he is ignorant of their implications, unless he intends the literal meaning of his statement." [Al-Fawakih Ad-Dawani]

Ibn ‘Uthaymeen said in relation to a young Bedouin who lived in the desert, where ignorance prevailed, and reached the age of 20 without fasting throughout his entire life, "It is not incumbent on him to do anything in this regard because he is excused for his ignorance." [Liqa’ Al-Bab Al-Maftooh]

In case this woman is excused for her ignorance, which we rule out as aforesaid, we believe that it is more prudent for her to fulfill her vow if she can do so. 

Allaah Knows best.