Why is Yosef called the Ben Zekunim? (Isn't Binyamin younger)

Let's begin with the verse, Genesis 37:3:

וְיִשְׂרָאֵל אָהַב אֶת־יֹוסֵף מִכָּל־בָּנָיו כִּי־בֶן־זְקֻנִים הוּא לֹו וְעָשָׂה לֹו כְּתֹנֶת פַּסִּים
Now Israel loved Joseph more than any of his other sons, because he was a ben zekunim to him, and he made a richly ornamented robe for him. (NIV)

The major English translations take ben zekunim as "son of his old age", "born to him in his old age" or similar. However, as just about everyone has pointed out, Jacob already  had a son who was even younger: Benjamin. Why wasn't Benjamin also beloved as a son of his old age? In fact, later Benjamin himself is called a yeled zekunim who "his father loves very much"  (Gen 44:20

For that matter, asks Ramban, what about the other ten sons? They, also, were born when Jacob was very old. Technically, even Reuben was a "son of his old age."

Onkelos addresses the problem with some slight of hand. Instead of reading "son of old age", he reads "wise son", likely drawing on the use elsewhere of zkn to denote wisdom. Tellingly, when Benjamin is later called a yeled zekunim Onkelos reads it literally. 

On our verse, Rashi first gives the literal translation then, likely due to the problems we have already noted, gives Onkelos's suggestion.  The idea here is that Jacob loved Josef most because he was the smartest son. If ben zekunim simply means "smart son" then we can't ask why Josef, and not Benjamin, was given that appellation.

Other possibilities:

Ben zekunim is a title, referring to the son who attends to his father, as opposed to the yeled zekunim which is a son born when his father is old. . If Joseph was his father's "body man" it helps explain why he was left behind when the other went with the sheep to Shchem. Earlier, Ibn Ezra writes that וְהוּא נַעַר אֶת־בְּנֵי בִלְהָה וְאֶת־בְּנֵי זִלְפָּה means that the sons of the maidservants treated Joseph like a servant, or na'ar (Abraham's servants are also called na'ar) Perhaps the sons of the lower status wives saw that Joseph acted as his father's servant and treated him that way, too? (I think this is one of the Targums. Not sure.)

Joseph was the ben zekunim, or youngest, for a relatively long time. He and his other ten brothers were born within seven years of each other; then eight years passed before Benjamin was born. Having gotten used to thinking of Joseph as the baby of the family, the brothers and their father found it difficult to shake this perspective after Benjamin arrived. (This might be Ibn Ezra. Its not mine)

A ben zekunim isn't merely the youngest son. Its also a youngest son who is best loved. In Jacob's view, Benjamin was always associated with the death of his favorite wife. As a result, he never bonded with his youngest son, or developed the affection for him that is typically associated with a ben zekunim. (I think this is Chizkuni)

Benjamin wasn't born yet. Support for this comes from Yaakov's own words when he asks his son: נָבוֹא אֲנִי וְאִמְּךָ וְאַחֶיךָ לְהִשְׁתַּחֲוֹת לְךָ אָרְצָה / Will I, and your mother, and your brother come and bow to you? The famous interpretation is that Jacob is pooh-pooing the dream on the grounds that Joseph's mother is dead. As Rashi puts it: Therefore, he said,“Will [we]come, etc.” Just as it is impossible for your mother, so is the rest meaningless.  However, its actually more straightforward to take Yaakov's words at face value. He mentions Joseph's mother because she, like the brothers, is still alive. And if Rachel is still alive, Benjamin hasn't been born yet. And if Benjamin hasn't been born yet, we don't have to ask  why Josef, and not Benjamin, was called the ben zekunim

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