Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
Whatever your mother did not fast out of heedlessness and carelessness, with no excuse, she does not have to make up for that. Whatever your mother did not fast because of an excuse such as menses, travel or sickness, she has to make up for it.
Answering your question, the Fatwa Center at Islam Q and A, states:
With regard to the fasts your mother did not observe until her marriage, this matter is subject to further discussion.
Whatever your mother did not fast out of heedlessness and carelessness, with no excuse, she does not have to make up for that.
Whatever your mother did not fast because of an excuse such as menses, travel or sickness, she has to make up for it. She should try hard to work out the number of days or what she thinks is most likely.
Almighty Allah knows best.
Source: www.islamqa.info