How old was Rebekah when she married Isaac?

In terms of a "timeline" (which is speculative, at best), ch.21 tells us the ages of A&S when Isaac is born, and that the casting out of Ishmael takes place when Isaac is weaned.

I know that the boy's putative age for this limit is sometimes pushed further than westerners are used to; but extending infancy (which is the practical effect of long-term nursing) does not strike me as an ordinary biblical value, and is not a cultural practice I imagine the patriarchs encouraged. So, I don't think of Isaac as being more than 5yrs old--old enough to just remember (when he got older) his first big party as the center of attention.

Such an age also would fit with Ishmael being still a late-teenager, a child, a boy/lad, a youth or young man. Without specific numbers, we can only guess; but it pushes the bounds of the context and language to think of him in his 20s. But the story of Ishmael fast-forwards in one verse (v21) to view him as an adult getting married.

Then, after the interlude (a few vv) of interaction and covenant between A and his neighbors, comes ch.22. And there, Isaac is referred to in v5 with the same word that Ishmael was called as a teenager. It's hard for me to put a nearly 40yrs old man into the category of "youth," even if he isn't married yet and he would eventually live to 180yrs.

I have a hard time thinking that the Israelite generation wandering in the desert, who were the first to hear the history in this form, and whose lifetimes were similar to our own, would have imagined a "youth" of 37yrs. It seems to me, we're meant to think of both sons as being about the same ages when these events happed to each of them.

Then, at the end of ch.22 we are reminded (as A is informed of some family-news from those left behind decades ago) that there is a family far away. Ch.23 begins recounting the death of Sarah. According to 25:20, Isaac is 40yrs old when he marries (37yrs when his mother died).

It is quite bizarre to suppose 22:23 (note of R's birth) is to be counted proximally from the age-notice of Sarah's death 2vv later (23:1), especially as the signs of her age (MUCH more than 4yrs) in ch.24 are so apparent. It is an example of forcing the text into an interpretive straightjacket, following "rules" with no flexibility.

Let us suppose Isaac is 15-20yrs when Abraham takes him up Moriah to be bound on the altar (we'll take the upper limit). Then there's 20yrs for chronologically determined events to happen between 22:19, and 24:67 when I&R marry. Moreover, the details of Nahor's family tree (end of ch.22) stretch back fully 45yrs or longer. Bethuel mentioned last is most likely the youngest son of eight. Did Nahor's children begin to be born in Haran and all after A moved on leaving them? There's still plenty of time--nearly 65yrs--until Isaac marries R.

So, let's play "timeline." Abraham is 75yrs (12:4). Nahor (add 15yrs?) begets Bethuel eighth estimating two years between brothers, (add 25yrs?) who begets R and Laban. That's 40 yrs. Add 25yrs to that, and R marries 40yrs old Isaac. Play around with the ages some, and it still all fits well. R clearly doesn't have to be (indeed can't be, given the evidence) a toddler child bride.