Now, let's go over the idea of accountability and where it is found in Scriptures.
The idea of accountability is simply the quality or state of being accountable, which is to be held responsible for your actions, and having to give an account or reason for such behavior.
As reasonable adults, we don't hold an infant or small child accountable the same way we do an adult or teenager, and the Bible alludes to this fact a couple occasions.
For before the boy knows how to refuse the evil and choose the good, the land whose two kings you dread will be deserted.
Isaiah 7:16 (ESV)
There is a period of time when a child simply does not know how to refuse evil and choosing good. Therefore, he is not able to provide a reason or explanation for his actions that would make him guilty of any crime.
And as for your little ones, who you said would become a prey, and your children,
who today have no knowledge of good or evil, they shall go in there. And to them I will give it, and they shall possess it.
Deuteronomy 1:39 (ESV)
Notice that this is the same depiction of Adam and Eve in the garden prior to the fall, it is a description of innocence, they don't have any knowledge of good or evil. Therefore, they are innocent as they cannot distinguish the two and only act ignorantly.
The particular age is known to God and I doubt it would be the same for every child, but there is an age of innocence for all children where they don't have the capacity to do otherwise and act on impulse and have no knowledge of right and wrong.
Now the concept of the Justice of God.
Far be it from you to do such a thing, to put the righteous to death with the wicked, so that the righteous fare as the wicked! Far be that from you! Shall not the Judge of all the earth do what is just?”
Genesis 18:25 (ESV)
Acquitting the guilty and condemning the innocent-- the LORD detests them both.
Proverbs 17:15 (NIV)
The Lord is JUST in all his judgments, and he will not condemn the innocent children for he will do what is right.
How is the character of God revealed in Christ towards children?
But Jesus called them to him, saying, “Let the children come to me, and do not hinder them,
for to such belongs the kingdom of God."
Luke 18:16 (ESV)