You're right, we're never given Benjamin's age so we can't link him to his brothers that way. We can, however, follow the chronology and the geography to know within about a year how long Jacob and Rachel were married. This isn't such a straightforward task, though, since the chronology of Genesis is confusing. At one point a decade passes between 2 verses (Genesis 33:17-18), then the narration continues forward without mentioning Joseph being sold by his brothers.
Then a few chapters later (Genesis 37) it goes back to that point (at the end of the fast-forwarded decade) and picks up the narrative from a Joseph's perspective and shows the events that led to Jacob abandoning his house in Succoth.
Knowing they arrived in Succoth when Joseph was 6 and that they left when he (and Dinah) was 17, we can say they were married about 25 years when she died. Here's how we get that number:
- Jacob worked for his uncle Laban 7 years, then married Leah & Rachel
- He worked another 7 years for Rachel
Now they've been married 7 years.
- He worked the next 6 years for his uncle's flocks, then he left Haran
Now they've been married 13 years. Joseph and Dinah are 6 years old.
- Jacob stayed in Succoth, built shelters for his livestock, then he built a house there and settled down to raise his 12 kids
- As Jacob's sons got older they worked with the flocks. Jacob's flocks grazed as far south as Hebron, because "the land where they [Jacob & Esau] were strangers could not support them because of their livestock" (Genesis 36:7)
- When Joseph was 17 years old, he went to check on his brothers, and they sold him into slavery. Then they let their father Jacob think Joseph had been killed by a wild animal.
It's now been 11 years since they left Haran (Joseph was 6, now is 17. Dinah is also 17). So now they've been married 24 years.
- Jacob tore his clothes and refused to be consoled and said, "I shall go down into the grave to my son in mourning" (Genesis 37:35)
- Judah did not approve of his brothers' dealings with Joseph, and lying to their father letting him think Joseph was dead. Judah left home, went to the area of Adullum, got married and had some kids (Genesis 38:1-5)
- Jacob, distraught beyond measure, packed up the family and they left their house in Succoth, arriving first in Shechem where he bought a plot of land to pitch his tent
- This is where Dinah (17) was sexually violated, then her brothers killed all the men of the town and the family had to flee. God told them to go to Bethel (Genesis 35:1,5)
- The family repented, buried their idols and moved to Bethel. When they came to Bethel, God said to Jacob "Be fruitful and multiply; a nation and a company of nations shall proceed from you, and kings shall come from your body" (Genesis 35:11)
- Joseph was Jacob's only child with Rachel, and he was presumed dead. But Rachel was pregnant with another son, and as they journeyed south from Bethel and were a short distance from Bethlehem, Rachel went into labor. Benjamin was born, but Rachel didn't survive
We're not told exactly how long after they left Succoth that Benjamin was born, but it was likely in the first year or two. We know this because Benjamin was a father when they went into Egypt 22 years after leaving Succoth (Joseph 17 years old - 39 years old)