From Whence The Ten Years Before Yitzchak and Rivkah Began To Try To Have Children?

Based on a discussion on this recent parshablog post about Rivkah's age of 3: Is this just our innate moral sensibilities that guide us to this assumption that they waited ten years, or is there something in the text that directs this?

To recap: Rashi says he took Rivkah 3 years after Akeidat Yitzchak, the first time such a marriage was possible. And Rashi also says that there was a 20 year gap, 10 years until she could possibly have children and another 10 years of trying.

Firstly, while we might conveniently cite it as a basis that they waited 10 years for nisuin and thus consummation, this is not really within the text of Rashi itself. In parshat Toledot, on the pasuk stating Yitzchak was 40 when he took Rivkah, Rashi writes:

(כ) בן ארבעים שנה -
שהרי כשבא אברהם מהר המוריה נתבשר שנולדה רבקה, ויצחק היה בן שלשים ושבע שנה, שהרי בו בפרק מתה שרה, ומשנולד יצחק עד העקידה שמתה שרה שלושים ושבע שנה, ובת תשעים הייתה כשנולד יצחק, ובת מאה עשרים ושבע כשמתה, שנאמר (כג א) ויהיו חיי שרה וגו', הרי ליצחק שלושים ושבע שנים ובו בפרק נולדה רבקה, המתין לה עד שתהא ראויה לביאה שלוש שנים ונשאה:

This is where he makes the calculation as to their respective ages, and he says that she was three years old ונשאה. (This based on Bereishit Rabba 57:1.) It could be he is being inexact and means kiddushin rather than nisuin, but it is also quite possible that Rashi envisioned consummation having occurred some time before 10 years had elapsed.

A bit later in the same perek, when the pasuk says that Yitzchak was 60 when Rivkah had Yaakov and Esav, Rashi writes:

בן שישים שנה -
עשר שנים משנשאה עד שנעשית בת שלוש עשרה שנה וראויה להריון ועשר שנים הללו צפה והמתין לה כמו שעשה אביו לשרה, כיון שלא נתעברה ידע שהיא עקרה והתפלל עליה. ושפחה לא רצה לישא, לפי שנתקדש בהר המוריה להיות עולה תמימה:

again referring to it as nisuin.

So moral sensibilities might not even come into play.

But aside from that, how do we get this 10 year gap until they begin to try to have kids. One thing I would suggest is that we see that Chazal maintained that medically, above a certain age, a girl is incapable of becoming pregnant, and early within that time, she could become pregnant but would die as a result. (The gemara in Yevamot either requires or permits birth control under these conditions, depending on if you ask Rashi or Tosafot on this point.) So that certainly feeds into it.

But the aforementioned Rashi gives the reasoning, based on pesukim. In Bereishit 25, we read:

כ וַיְהִי יִצְחָק, בֶּן-אַרְבָּעִים שָׁנָה, בְּקַחְתּוֹ אֶת-רִבְקָה בַּת-בְּתוּאֵל הָאֲרַמִּי, מִפַּדַּן אֲרָם--אֲחוֹת לָבָן הָאֲרַמִּי, לוֹ לְאִשָּׁה.	20 And Isaac was forty years old when he took Rebekah, the daughter of Bethuel the Aramean, of Paddan-aram, the sister of Laban the Aramean, to be his wife.
כא וַיֶּעְתַּר יִצְחָק לַיה לְנֹכַח אִשְׁתּוֹ, כִּי עֲקָרָה הִוא; וַיֵּעָתֶר לוֹ ה, וַתַּהַר רִבְקָה אִשְׁתּוֹ.	21 And Isaac entreated the LORD for his wife, because she was barren; and the LORD let Himself be entreated of him, and Rebekah his wife conceived.

and she could not be established as an akara until 10 years of trying. The same is encoded in halacha (not in practice today), of taking a different wife, in order to fulfill peru urevu, if ten years passed. And we see Avraham similarly waiting for Sarah. And we see a bit later in the same perek that 20 years had passed:

כו וְאַחֲרֵי-כֵן יָצָא אָחִיו, וְיָדוֹ אֹחֶזֶת בַּעֲקֵב עֵשָׂו, וַיִּקְרָא שְׁמוֹ, יַעֲקֹב; וְיִצְחָק בֶּן-שִׁשִּׁים שָׁנָה, בְּלֶדֶת אֹתָם.

26 And after that came forth his brother, and his hand had hold on Esau's heel; and his name was called Jacob. And Isaac was threescore years old when she bore them.

because he was now 60, and they started this at 40. So why 20 years and not 10? This now works well with the assumption that they married when she was 3. The first ten years was waiting until she reached maturity.

This Rashi is partly based on Pirkei deRabbi Eliezer, perek 32, pictured to the right. (So says Judaica Press.) Although if you compare It states that:

Rabbi Yossi says that for three years Yitzchak was in mourning for Sarah his mother. After three years, he took Rivkah, and forgot the mourning of his mother. From here you learn that before a man takes a wife, his love goes after his parents. Once he takes a wife, his love goes after his wife. And he cites Bereshit, al ken yaazov ish et aviv veEt imo vedavak beIshto. Now does a man who takes a wife abandon the commandment of kavod {kibbud av vaEm}? Rather, the love of his soul attaches after his wife, for it states vedavak beIshto. Rabbi Yehuda says that for 20 years Rivkah was an akarah {barren}. After 20 years he took Rivkah and went with her to Mount Moriah, to the place where he was bound, and prayed for pregnancy, and was answered.

And cites that pasuk.

I wonder whether Rabbi Yossi and Rabbi Yehuda are meant to be combined. It is possible. But on the other hand, Rabbi Yehuda is saying that she was barren for 20 years, something which does not seem to apply to a girl under the age of 12. Indeed, even within Rabbi Yossi, the three years waiting is not until she is capable of being married, at least not explicitly, but rather is just spent in mourning for Sarah his mother. Though 3 years waiting certainly does seem to connect with the other idea Rivkah's age. So maybe that is the intent of the midrash here, or maybe not.