"Elleh Toldoth Ya'aqob, Yosef" (these are the generations of Jacob, Joseph). When the Torah says these are the generations of "Ya'aqob", it would seem appropriate to then list all his sons. Instead the Torah only mentions "Yoseph". The simple meaning is that all the brothers' characteristics were included in Yoseph Hasaddiq, 'a"h.
We know that Joseph was Ya'aqob Abinu's favorite son and even though Reuben was acually the first born, Joseph was the one who carried the distinction of being the son of Ya'aqob Abinu. According to the Midrash, Yoseph Hassadiq is compared to Ya'aqob Abinu, 'a"h, because they were alike. Whatever befell Ya'aqob Abinu, also befell Joseph. For instance, just like Ya'aqob Abinu, 'a"h, was chased away by his brother 'Esau who wished to kill him, so too Yoseph Hassadiq was chased away by his brothers. Just as Ya'aaqob Abinu was elevated through a dream (of the ladder) so too Yoseph was elevated by his dream.
When the Torah states that Joseph was "Ben Zequnim Hu Lo" (the son of his old age), it is referring to the fact that they looked similar and does not mean that Joseph was the youngest, because, in fact, Benyamin (Benjamin) was six years younger than Yoseph Hassadiq, 'a"h.
(See Rabbenu Behayye, 37: 2)
