How Old Was Rivkah?

One of the less talked about parts of Chumash is child marriage.  For example, an unmarried Kohen Gadol must take a young virgin in marriage.  Imagine the optics - a 60 something year old man in full priestly regalia under the chuppah with a 10 year old.  Not something I'd want to see on CNN.
Another one comes up in this parsha.  At the beginning of Toldos we are told that Yitchak Avinu, a"h, was 40 years old when he married Rivkah Imeinu, a"h.  Rashi, based on Seder Olam, does the math for us.  Yitchak Avinu was 37 at the time of the Akeidah, based on the assumption that Sarah Imeinu died right after from the shock of hearing about it.  At that point, Avraham Avinu, a"h, learns that his cousin Rivkah has been born.  Therefore, concludes Rashi, Rivkah is three when she marries Yitzchak.  The Torah then tells us that Yitzchak Avinu was 60 when the twins were born which means he and Rivkah tried for 20 years.  Rashi, in order to remain consistent with the halacha that a man should divorce his wife if she hasn't gotten pregnant after 10 years of trying, tells us that from age 3-13 we wouldn't have expected Rivkah to get pregnant and therefore it was only 10 years of real infertility that counted until Eisav HaRasha and Yaakov Avinu, a"h, were born.
Bottom line: 40 year old Yitzchak Avinu married 3 year old Rivkah Imeinu.  It sounds creepy until you factor in that this 3 year old could draw water for camels and ride them all by herself.
Although there is no debating Rashi's central importance in understanding Chumash, it is sometimes forgotten that while he is the first voice, there are many others to be heard after his.  This is an example of where that's important. 
There are, in fact, many commentators who don't accept Rashi's version of the chronology.  Amongst the reasons brought are (1) there's no reason to believe that Avraham Avinu received the news of Rivkah's birth immediately.  She could have been much older by the time the family telegram reached him. (2) Yes, people matured faster back in the days before permanent adolescence meant not growing up until you turned 35 but three year olds have always been three years olds.  They can't go out and draw water by themselves, they're not strong enough to fill a trough for camels and there certainly can't ride them by themselves.  Rivkah Imeinu could not have been 3 years old for this story to have happened the way the Torah tells us.
Rav Michael Hattin, formerly of Yeshivat Har Etzion, notes his and brings a great explanation of what led Rashi to comment on the story and I would like to share it with you.
He starts by noting that Rashi was no fool.  He knew that taking Seder Olam literally meant insisting that a three year old could perform all sorts of tasks that were clearly beyond her.  He goes a step further and asks about Yitzchak Avinu's age at the Akeidah.  Although again Rashi uses traditional sources to determine that he was 37, Rav Hattin again shows that this was not likely.  After all, if Yitzchak Avinu was an adult at the height of his strength, then the Akeidah wasn't just a test for Avraham Avinu but for him as well.  The Torah, however, never portrays the Akeidah as a big test for Yitzchak Avinu, just his father.  In all likelihood he was much younger, probably still just a child.
If that's the case, why does Rashi insist that Yitzchak Avinu was 37 and Rivkah Imeinu was only 3?  This can be tied into Rashi's comment from the Midrash about the wording around Sarah Imeinu's age at the time of her death. Famously, Rashi notes the importance of each appearance of the word "year" in that verse.  This concept gets extended further.  Yitchak Avinu, despite being a young boy, acted with the maturity of a 37 year old when it came time to lie on the altar for his father and submit to being sacrified.  A normal boy would have run screaming into the woods or called the Canaan Child Protection Services on his cell phone.  Not Yitzchak Avinu.  Despite his age, he understood the importance of what was going on and acted appropriately.
Similarly for Rivkah Imeinu, one needs to note that the marriages of our forebears were not simple shidduchim but a joining of mammoth spiritual personalities.  A normal girl of marriageable age might have been influenced to marry Yitzchak Avinu for the promise of the wealth he possessed.  Instead, Rivkah Imeinu looked at the situation with a level of honesty found only in young children.  For her, it was Yitzchak Avinu's godlus that sealed the deal, not a sense of personal interest. 
Thus Rashi, by bringing these midrashim and making statements that superficially seem untenable was actually giving us a deep insight into the greatness of both Yitchak Avinu and Rivkah Imeinu.