How old was Joseph when he became prime minister of Egypt?
In the book of Genesis Joseph becomes second in charge under only Pharaoh at the age of 30, as stated in Genesis 41:46 :
And Joseph [was] thirty years old when he stood before Pharaoh king of Egypt. And Joseph went out from the presence of Pharaoh, and went throughout all the land of Egypt.
Genesis 41:46
As you can see from the above passage Joseph was 30 years old when he stood before Pharaoh and became prime minister of the land of Egypt.