The age of accountability is the idea that there is an age where an individual becomes responsible for being able to distinguish between right and wrong. There is no biblical statement concerning this. Nevertheless, children are all under the Fall of Adam. Rom. 5:12 says, "Therefore, just as through one man sin entered into the world, and death through sin, and so death spread to all men, because all sinned." Also, In Psalm 51:5 David said, "Behold, I was brought forth in iniquity, and in sin my mother conceived me." Therefore, we are all born under the curse of the Fall.
The atoning work of Christ is received by faith. But, can an infant believe? Psalm 22:9 says, "Yet You are He who brought me forth from the womb; You made me trust when upon my mother’s breasts." So, though children from the conception are under the Fall of Adam, God is able to make them believe and trust in him.