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.

Les teaches that Rebecca conceived twins, Esau and Jacob, and before they were born—before either had done anything good or evil—God's purpose of election was declared. This was not based on works or what man deserves, but on the sovereign God who calls. Les illustrates that while the invitation 'Whosoever will may come' is offered, those who respond look back and find they were chosen from before the foundation of the world.

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;)

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.