How could God say the elder would serve the younger before Jacob and Esau were even born?

God declared it before their birth because His purpose of election would stand, not based on works, but on Him who calls. Operating in foreknowledge, seeing the end from the beginning, God knew Esau would be destitute of faith, so the elder would serve the younger.

Les explains that this statement came while Jacob and Esau were still in Rebekah’s womb, neither having done any good or evil, so that the purpose of God according to election might stand, not of works, but of Him that calls. It was contrary to Middle Eastern custom, where the firstborn twin would be the eldest son and rightful heir of the inheritance. This flew in the face of what Isaac and Rebekah understood, because the younger, second twin, Jacob, would be the ascendant one, the one through whom the promises would come. Les says it is against human reason, but he views it through God’s foreknowledge: before those lads were ever born, seeing the end from the beginning, God knew Esau would be destitute of faith.

Romans 9:11

(For the children being not yet born, neither having done any good or evil, that the purpose of God according to election might stand, not of works, but of him that calleth;)

When was God's purpose of election declared for Jacob and Esau?

God's purpose of election for Jacob and Esau was declared before they were born, while still in Rebecca's womb, before either had done good or evil. Scripture states this was so election might stand not of works but of Him who calls, chosen from before the foundation of the world.