When Rashi gathers a collection of verses and makes inferences from numerical computations we say Rashi uses the Spreadsheet method. The Table below shows it reasonable that Rivkah was 15 years old when she married Isaac. After reviewing the table we will explain its rows line by line.

================== LIST690b ==========================
List of verses on Rivkahs age when she married Isaac
======================================================
ID VERSE    Isaac Rivkah Event at that age of Isaac
== ======== ===== ====== =============================
A  Gn17-17  0            Sarah is 90 when Isaac born  
   ------   ----  -----  -----------------------------
B1 Gn22-01  25-30 0-5    Isaacs age at time of Akaydah 
B2 Gn22-20a 25-30 0-5    Bethuayl had been born
B3 Gn22-23a 25-30 0-5    Rivkah had already been born
   ------   ----  -----  -----------------------------
C  Gn23-01  37    12     Sarah 127 at time of death   
D  Gn25-20a 40    15     Isaac Marries Rivkah  
E  Gn25-26d 60    35     Pray for children;Twin births
======================================================

We now explain each row.
• Row A cites verse Gn17-17 that Sarah was 90 when Isaac was born.
• Rows B1-B3 mention 2 events that happened more or less at the same time between the birth of Isaac and the death of Sarah.
  • The Akaydah, the offering of Isaac
  • Rivkah had already been born to Bethuayl
  No verse indicates the age of Isaac and Rivkah at the time of the Akaydah. We are free to guess. In the table above we assume Isaac was 30 and Rivkah was 5. An alternate computation is presented below.
• Row C cites verse Gn23-01 that Sarah was 127 when she died. Using spreadsheet methods we infer that Isaac was 37 at the time of Sarah's death since Sarah was 90 when Isaac was born. We also can infer that Rivkah was 12 at the time of Sarah's death (using the Isaac=30 and Rivkah=5 assumption)
• Row D cites verse Gn25-20 which states that
  • Isaac was 40 when he married.
  • We know he was 37 when his mother died.
  • We conclude that he married 3 years after his mother's death
  • Hence Rivkah married 3 years after Sarah's death and was 15 according to our assumptions that Isaac was 30 and Rivkah was 5 at the time of the Akaydah.
• Row E cites verse Gn25-26 which states that Isaac was 60 when he had his 2 children. Since he was 40 when he got married we infer that 20 years had elapsed. It follows that Rivkah was 35.

An alternate spreadsheet is presented below. Here we assume that Isaac was 37 at the time of the Akaydah and Sarah died immediately from the shock of hearing that her son was offered. We also assume that Rivkah had just reached adulthood and was 12.

================== LIST690b ==========================
List of verses on Rivkahs age when she married Isaac
======================================================
ID VERSE    Isaac Rivkah Event at that age of Isaac
== ======== ===== ====== =============================
A  Gn17-17  0            Sarah is 90 when Isaac born  
   ------   ----  -----  -----------------------------
B1 Gn22-01  37    12     Isaacs age at time of Akaydah  
B3 Gn22-23a 37    12     Rivkah had already been born
   ------   ----  -----  -----------------------------
C  Gn23-01  37    12     Sarah 127 at time of death   
D  Gn25-20a 40    15     Isaac Marries Rivkah  
E  Gn25-26d 60    35     Pray for children;Twin births
======================================================

Advanced Rashi: There are many more spreadsheet computations possible. The reader is free to assume that Isaac was any age between 0 and 37 at the time of the Akaydah and is free to assume any age for Rivkah. By playing with the spreadsheet the serious reader will learn to distinguish between what is textually given and what is inferred.

Rashi literally says Isaac did not marry Rivkah immediately but waited until she was ready for intimacy 3 years. Some interpret the underlined passage literally. There is a Talmudic dictum that relations with a female child under 3 is not considered a male-female relationship. Hence Isaac married Rivkah at 3 and did not consumate the marriage till she was 12.

But I suggest that waited until she was ready for intimacy 3 years means waited 3 years after puberty. In other words at puberty she is first ready for intimacy and he waited 3 years during which she is ready of intimacy. The purpose of the 3 year wait was to allow Rivkah experience in contacts with men. According to this, Rivkah married at 15.

We now explore another nicety in Rashi interpretation. Rashi on Gn25-26d states Rivkah married at 3; Isaac waited 10 years till she was capable of birth at 13; he then waited 10 years for children since under Jewish law if a couple does not have children in 10 years they must get divorced and try with others. The Rashi commentators insert the reference to the Talmud, Tractate YeVaMoth 64a

There are several problems with this Rashi. First, Rivkah was capable of birth at 12, not 13. So there is an extra year not accounted for.

More serious is the fact that the Talmud which Rashi is alleged to cite does not say any of this. On the contrary the Talmud states that both Isaac and Rivkah were barren and hence the 10 year rule did not apply (They needn't get divorced).

Since Rashi contradicts the Talmud I would prefer to amend Rashi to be consistent with the Talmud. I would explain There is a 20 year gap between the marriage at age 40 and the birth of twins at age 60. Since both Isaac and Rivkah were barren I would say that Isaac prayed for 10 years and was cured of his barrenness. Rivkah then prayed 10 years and was cured of her barrenness.

Sermonic points: I would particularly focus on the Rashi comment He waited for her until she was capable of relations for 3 years which we have interpreted to mean that he waited 3 years of puberty to age 15. This is a statement about marriage suitability. Judaism does not allow serious relations, full or partial intimacy,prior to marriage. However men and women should have sufficient verbal contact with members of the opposite gender so as to be able to function and be aware of what is normal and what is not normal.