With the exception of Christ Jesus, all of us are born in sin.¹  None of us, however, are held accountable for our sin until we reach the "age of accountability."  Although not specifically mentioned in the Bible, the "age of accountability" is alluded to.

When we come to a point in life whereby we are capable of knowing that there is a God, and also capable of turning to Him in repentance through faith in Christ Jesus as Lord and Savior, then, at that point, we are held accountable for sin - prior to that, we aren't.

Many never reach the "age of accountability" due to death at a very young age, or having been born with brain damage which makes it impossible for them to know that there is a God, and also impossible for them to turn to God in repentance through faith in Christ Jesus as Lord and Savior.   

Those who die having never reached the "age of accountability" go to be with the Lord in heaven upon their death.²  King David, who wrote most of the Psalms in the Bible, believed this; for he said of his baby son who had died: "While the child was alive, I fasted and wept; for I said, ' Who can tell whether the LORD will be gracious to me, that the child may live?'  But now he is dead; why should I fast?  Can I bring him back again?  I shall go to him, but he shall not return to me."³

Bible References:

¹ (1 John 3:5; Psalm 51:5)

² (2 Corinthians 5:8)

³ (2 Samuel 12:22-23)