Of all the generations in Genesis, the age at which each became a father/begot a son/twins were born is almost always shown. The only exceptions are the last two, Jacob and Joseph.
Why might that be the case?
Note: Jacob’s and Joseph’s ages for becoming fathers can be calculated, but only when considering unique data like Jacob’s “years as a wayfarer amount to 130” (Genesis 47:9) and “Joseph was 30 years old when he entered the service of Pharaoh, king of Egypt" (Genesis 41:46).
Ussher provides answer half of your question in his work, Annuls of the World:
Jacob was ninety-one years old when Joseph was born, and consequently, seventy-seven years old when he first began to serve Laban. This may be deduced, for Jacob was a hundred and thirty years old when he first stood before Pharaoh at the time when the seven years of plenty were past and two years of famine were over. {Ge 45:6 47:9} Joseph was then thirty-nine years old. He was thirty years old when he first came before Pharaoh, just before the seven years of plenty {Ge 41:32, 46}.
