according to their birth
Exodus 1:1-4
1
Now these are the names of the children of Israel, which came into Egypt; every man and his household came with Jacob.
2
Reuben, Simeon, Levi, and Judah,
3
Issachar, Zebulun, and Benjamin,
4
Dan, and Naphtali, Gad, and Asher.
Genesis 43:33
And they sat before him, the firstborn according to his birthright, and the youngest according to his youth: and the men marvelled one at another.