For many men in Genesis, ages for becoming fathers and ages at death are given.
However, there are also some rather unique ages given. For example:
Joseph was 30 years old when he entered the service of Pharaoh, king of Egypt Gen 41:46a TNAB
Why might an age like this be given?
Note: Other unique ages given include Ishmael’s age of 13 when he was circumcised (Gn 17:25) and Abram’s age of 86 when Hagar bore him Ishmael (Gn 16:15).
While it is true that people's ages at significant events are often given, Joseph's age when he began to rule is necessary to establish the chronology of Jacob's life. We know that Jacob was 130 years old when he came to Egypt, and that he came when there were 5 years of famine left. That is, after Joseph began to rule there were 7 years of plenty and 2 years of famine, and then his family came - 9 years after he began to rule. That is, Jacob's 130th year coincided with Joseph's 39th year - Jacob had Joseph when he was 91 years old. We know that Jacob had Joseph at the end of his contract with Laban, 14 years after he had fled from home - thus he fled home 91 - 14 = 77 years old. The author seems to have purposely left this kind of information in the text in order for the reader to be able to piece together such chronological details.
Those ages are the times of significant events in their lives. While the age of the birth of the next in the godly line1 and death are certainly important, these are not the only events that matter. For example, Noah was 600 years old when the Flood came (Genesis 7:6).
1Not necessarily their first born. Adam is listed as 130 years old when Seth was born. However, Seth is the third named son (Abel and Cain being born and grown prior). It stands to reason that other sons were born between the expulsion and Seth whose names were not recorded in the records. Genesis 5:4 says that there were other "sons and daughters."
