This question is one of the Muslims’ favorites. They just love it, just like they love Luke 19:27 because they think they have found a grave defect in the
Bible, when really all they have done is to misread the text, or maybe more to the point, they haven’t read the text at all.

Muslims insist that Rebecca was three years old when she married Isaac, no doubt an attempt to deflect away from the fact that Aisha herself claimed in the hadith to be six years old when she was betrothed to Muhammad, and nine years
old when he consummated the marriage. To draw attention away from Aisha, they claim that the Bible gives Rebecca’s age as three years old, but if you read the relevant passages you will find there is no such indication of her age. Where
then do the Muslims get this, if not from the Bible? The key is that they misread the text by making the unfounded assumption that Abraham’s binding of
Isaac on the altar, Rebecca’s birth, and Sarah’s death all happened at the same time, thereby compressing time by a decade or more. In this way they are able to remove the gaps in the Bible narrative to arrive at an age of three for Rebecca
at the time of her marriage. But sometimes, it is wise to use a little common sense when trying to cut down a time line to arrive at the results you want.
What the Muslims have done makes no logical sense.

Here is a crude example of how their thinking works: “When I was five years old my parents moved to a new apartment across town to be closer to the school I would attend. Later, I got my driver’s license.” Muslims would use this story t0
prove that I got my driver’s license when I was five years old. Never mind that at five years old my little feet would not have reached the pedals. As silly as
this is, it is exactly what Muslims have done in order to claim that Rebecca was three years old when she was married to Isaac.

But there is more than misreading the Bible. They also draw on the writings of Medieval Jewish Rabbis from the 11th century to make their case. More on that later.

To develop a logical conclusion we should consider four main areas: biblical facts, Islamic traditions, age of consent, and Rabbinic traditions.

BIBLICAL FACTS: Where did the idea that Rebecca was only three years old when
she was married to Isaac come from if it is not in the Bible? Let us first review the facts that we are able to ascertain from the Bible:

(1) Sarah was 90 when Abraham was 100 [Genesis 17:17]. (2) Abraham was 100 when
Isaac was born Genesis 21:5 Sarah died at age 127 Genesis 23:1-2
Isaac was 40 when he married Rebecca [Genesis 25:20]

from these four facts we can infer the following two points:

(5) Sarah was 90 when Isaac was born. [Conclusion drawn from (1) and (2) above.
(6) Isaac was 37 when his mother, Sarah, died [127-90=37]

These six facts do not give us enough information to determine Rebecca’s age when Isaac married her.

TRADITIONS To attempt any calculation of Rebecca’s age, we must make assumptions
not substantiated by Scripture. This is to deny what the Bible says in order to uphold the sought-after results. Jesus warned about making such assumptions in Matthew 15:6, “So for the sake of your tradition you have made void the word of
God.” In other words, to prove that which is not contained in Scripture, you must add something that is not scriptural, such as your own conjecture. The assumptions in the traditions are conjectural and not scripture, and thus not the word of God, but the word of men.

Tradition makes certain assumptions that are not confirmed in Scripture, that the following three events happened at the same time or within a few weeks of each other:

• The binding of Isaac on the sacrificial altar in Moriah [Genesis 22:9] –
  Abraham informed of Rebecca’s birth [Genesis 22:20-23], quite a stretch of the imagination, to say the least. – Sarah’s death at age 127 [Genesis 23:1-2]

If these three assumptions all did happen at the same time, or within a few weeks, then we can make two more necessary inferences:

(7) Since Isaac was 37 at his mother’s death, this means he was 37 when Rebecca was born. (8) Since Isaac was 40 when Rebecca was born, this means Rebecca was
3 years old when Isaac married her.

(In a similar manner, it was proved that I got my driver’s license at age 5.)

However, the assumptions made in points (7) and (8) are entirely dependent on the assumption that Rebecca was born at the same time that Sarah died. The assumptions in (7) and (8) are spurious because they are based on non-biblical
tradition and not on what the Bible says. The gaps in the narration have been stripped away. If a decade or so had passed between Rebecca’s birth and Sarah’s death, then Rebecca would have been 13. (A young woman [Genesis 24:16]) and not
a three-year old baby. One would not refer to a three-year old baby as a “young woman,” as the Bible does.

To make the case that Rebecca was only three years old at the time of her marriage to Isaac, Muslims have to accept non-biblical traditions and ignore or reject what the Bible says. You might as well forget what the Bible says and make up whatever you want.

Nowhere does the Bible tell us that Abraham’s binding of Isaac, Rebecca’s birth, and Sarah’s death all happened at the same time: – There is no indication of Isaac’s age at the time Abraham bound him on the altar, except to say that Isaac was called a “lad” or a “youth” by Abraham. Would one call a 37-year old man a
“youth”? – Abraham lived at Beersheba after the binding of Isaac. We are not told how long Abraham dwelt at Beersheba. – It was sometime “after these things”
[Genesis 22:20] that Abraham was told that Milcah had borne eight children to his brother, Nahor. We also do not know how long it took this news to reach Abraham. – Notice that when Genesis 22:23 says that “Bethuel begat Rebecca”, this was not part of the news that Abraham received. He was told that Nahor begat Bethuel. The birth of Rebecca was a generation later. We do not know how old Bethuel was when Abraham heard of his birth, nor do we know how old Bethuel
was when Rebecca was born.

The Muslims have taken all of the events above, ignored a generational gap, and compressed 25 years or so down to the same point in time.

AGE OF CONSENT That Rebecca was not a baby of three years is proven by the fact that she had reached the age of consent. This is one of the things that Muslims must ignore because it does not work with their narrative. One does not ask a
three-year old baby if it will agree to marry someone. Consider the evidence:

• Abraham made his servant swear an oath [Genesis 24:2-9] that “if the woman” (not if the baby) “is not willing to follow you, you will be free from this oath.” [Genesis 24:8]. In other words the woman (not a baby) must be willing to
  go to Isaac and that the marriage was to be at her consent. -Rebecca’s family also asked her if she would consent to go. Her mother and her brother, Laban, asked Rebecca if she would be willing: “And they said, ‘We will call the girl and ask her to speak her wishes by her own mouth.’ Then they called the girl and said, ‘Will you go with this man?’ And she said, ‘I will go.’ ” [Genesis 24:56-59] This does not prove her age but it does prove that she had reached the age of consent.

Then Rebecca mounted a camel and followed the man, together with her “nurse” [Genesis 24:59]. Her nurse was named Deborah. [Genesis 35:8]. The word nurse means a maidservant, or handmaiden, who was also responsible for her education.
This does not imply that this person was a nurse to a baby as Muslims insist. A lady of rank is, on her marriage, always accompanied by a nurse, who, as a confidential servant, is held in great regard by the family.

It is hard to image a three-year old baby mounting a camel, as it is equally hard to imagine a baby drawing a few hundred pounds of water from a well sufficient for 10 or 20 camels.

RABBINICAL TRADITION By the 1300s, Islam had spread across North Africa and into Spain. Muslims rely on the speculation of several rabbis, writing about 2,000 years after the events in question. These rabbis used unscriptural assumption to arrive at an age for Rebecca, and it is their writings that are the basis for Muslims’ assertion that Rebecca was only three years old. Their writings, however, are not accepted as scriptural. I suspect that they arrived at their
conclusions at the behest of their Muslim overlords.

I submit these as reference for anyone who might want to check it out for themselves:

Genesis Rabba (Midrash Rabbah) Rabbi Abraham Cohen de Herrera (1570-1635 AD)
Rabbi Rashbam (1085-1158 AD) Targum Jonathan