Genesis 37:3-4, 12-13a, 17b-28a (NRSVCE)
Jacob loved Joseph more than his brothers because he was “the son of his old age.” The phrase is interesting. It could simply be referring to how a father loves his youngest, because the older he gets the more precious children become to him. But if that’s the case, surely Benjamin would be the “son of his old age” and not Joseph, as Benjamin was Joseph’s younger brother.
The title could also be referring to Joseph being wise himself. He is old in mind, being wise, but young in age, and thus he is “the son of his old age.” There are a great many Biblical commentaries out there which use this explanation. But the story, at least in today’s reading, paints a picture of Joseph that is not very wise at all. In fact, he seems almost oblivious to everything around him. He is the favored son, as he and his brothers all know, and he has dreams about how his brothers will all bow down to him (the dreams are in those omitted verses from today’s reading). He won’t stop talking about the dreams, and keeps reminding his brothers about how they will one day bow before him. They hate him for that. And then we see Jacob send Joseph out to find his brothers in the field, and Jacob wears his special robe (his “coat of many colors”), the sign of his position above his brothers. He is going to find his brothers, who hate him for his self-importance, wearing the sign of his importance. Surely someone who is wise would recognize the trouble he is stirring by stoking the tension with his brothers.
So it seems to me it’s probably nothing to do with wisdom, and probably nothing to do directly with Joseph’s age. He doesn’t seem wise in this context, and he’s not the youngest of Jacob’s sons. I do recall reading about how traditionally a father would choose one of his sons to accompany him in his old age, keeping him near at all times to counsel him and prepare him to take over headship of the family. Unfortunately, I can’t seem to find anything on that right now. But that fits a bit more nicely with the “son of his old age” title in the context of this passage. Joseph is clearly chosen to take over headship once Jacob is gone. That is why Jacob gives him the robe. And while Joseph’s brothers are all out busy in the fields, Joseph remains with his father, despite Joseph being a teenager here and more than capable of joining his brothers. So “son of his old age” could simply mean that Joseph was the son chosen by Jacob to be the favored son who will take his place. That Joseph was chosen for this makes sense too when you consider that Rachel was the favored wife of Jacob (over Leah), and Joseph was the firstborn son of Rachel. We’re not told how long it is between Joseph’s birth and Benjamin’s, but it’s possible that Jacob had already chosen Joseph to be the favored son before Benjamin entered the picture.
I’m not sure how much water that idea of “son of his old age” holds, but it seems to make the most sense of the context in which the title is given to Joseph here.