That was the true Light which gives light to every man coming into the world. John 1:9 Since those lost have no excuse it indicates that God has given them the same Grace, opportunities, and choices He has given the saved or else they could claim, "you are punishing us for laws that are impossible to keep."
That doesn't actually answer the question of when this grace is given unless you mean as soon as Jesus was born. John 1:9 does not say grace but light, and it does not say enlightenment, but light.Verse 10 says He was in the world, and the world was made through him, yet the world did not know him. (Romans 1:19-20). Verses 12-13 say
But to all who did receive Him, who believed in His name, He gave the right to become children of God, who were born, not of blood nor of the will of the flesh nor of the will of man, but of God.
So there is nothing about choice there. Receive does not always denote choice, but here explicitly applies to being believing. If one hears the gospel and believes it, they would never reject what they believe. The light that is given to every man is Jesus coming into the world to redeem a people for God from out of the whole world. It is not enlightenment to every individual.
I can't imagine being born in sin unable to meet the standards of God because it is impossible for me to do so and then being eternally punished for it.
It isn't about what we can and can't imagine. It is about what God says. We are all born into that condition, dead in our sins and trespasses, until/unless God quickens us to life. Some receive justice, some receive mercy. Who are you o man to argue against God who says He will have mercy on whom He has mercy, and compassion on whom He has compassion? The disconnect here is in not grasping the holiness and complete otherness of God as Creator and our obligation to the One who made us. I know you know these things to be true, but that is different than grasping them in a way that causes one to stop measuring God by their feelings and reactions and purely from the human perspective. God has no obligation whatsoever to His creatures. Everything He does for them---all of them---and especially mankind, considering how great our transgression, how far our fall, is pure grace and mercy. The food we eat---His mercy. The rain that falls---His mercy. Not being destroyed enasse long, long ago which is what we deserve,---HIs mercy. He owes no one anything. We all owe Him everything. We do not lie, or move, or have any being, outside of Him.
But the bible does not say that at all. It is pretty straightforward. It states that men are without excuse for rejecting God. And the results are deeper and deeper ignorance and darkness. Men cannot say, "well, your laws were impossible to keep." Or "You never gave me any Grace." On the face of it, to plunge a man into hell for all eternity when there was no way he could meet the standards God set is not what He would do. Any earthly parent who punished their children for failing to meet impossible standards would rightfully be deemed a bad parent.
You keep comparing God to man. Have you not heard, "His thoughts are higher than our thoughts, so high we cannot attain to them."
God sent Christ as our substitute, obeying all God's laws perfectly, because we cannot, then taking upon Himself the just punishment for
our sins, dying the death that we justly deserve, taking God's just wrath against our sin on the cross, for that very reason. But only being in Christ by grace and through faith, and that not of ourselves, but is a gift of God, are our sins atoned for. And only in Him are we justified before God and reconciled to Him. That He does not do this for all is not unfair, it is unequal. Just as choosing Israel over all other nations was unequal. God can do whatever He pleases with the same lump of clay (humanity, all descendants of the man of the earth) whether anyone likes it or not.