In Bible you won't find her exact age. But i think it's safe bet to say she was very young. She was just married and i in I AD that was happening to teenage girls.
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I can not find any passages in the Qur'an that give her age. But I think it is of value to see what is not mentioned.
It seems if she was much below or above the child bearing age at that time it would have been mentioned. So it is probably safe to say she was within what was a common age to have children at that time. We also know that she was not married. It seems she was not at an age older than the typical age girls of that time were married which would have been between the ages of 14 to 16.
so all we know for certain is she was very young. a safe guess would be between 14 and 16. I believed if she had a child before the age of 14 that would have been seen as an oddity in itself and noted. If she was above the age of 16 and still single during that era I believe that would have been noted as an oddity.
So it seems that her giving birth was nothing unusual except for the fact she was a virgin.
It is highly possible given many ancient customs at the time that Mary's age was likely between 13 and 15 years old at the time of Christ's birth: at that time this was considered a young adult's age, and was commonly the age range in which most girls were married.