Age of Accountability.

Gotquestions says, if a child dies before reaching the “age of accountability,” that child will, by the grace and mercy of God, be granted entrance to heaven. Is the concept of an age of accountability biblical? All people, including infants and children, are guilty before God because of inherited and imputed sin. Inherited sin is that which is passed on from our parents. In Psalm 51:5, David wrote, “Surely I was sinful at birth, sinful from the time my mother conceived me.” David recognized even at conception he was a sinner. The sad fact that infants sometimes die demonstrates that even infants are impacted by Adam’s sin, since physical and spiritual death were the results of Adam’s original sin. Each person, infant or adult, stands guilty before God; each person has offended the holiness of God. The only way God can be just and at the same time declare a person righteous is for that person to have received forgiveness by faith in Christ. God’s extending grace to those who cannot believe would seem consistent with His character. We can’t be dogmatic but it would seem that God does apply Christ’s payment for sin to babies and those who are mentally handicapped, since they are incapable of understanding their sinful state and their need for the Savior. God is loving, holy, merciful, just, and gracious. He loves children (Matthew 19:14).

Dr. Andy Woods says, In 2 Samuel 12, David's son from his illicit relationship with Bathsheba dies, and David mourns until he suddenly stops and says he cannot bring his child back, but he will go to him. This passage implies that David believed he would see his child again in the afterlife. This raises questions about Calvinism, which suggests that non-elect children go to hell. However, David's reaction doesn't align with this belief, leading some to think that children are covered by God's grace before a certain age, before they can make a decision.

The exact age is unknown, but many believe that young children are protected by God's grace until they can make a decision. It doesn’t fit right with what I know about God’s Grace who desires all to be saved and all to come to a knowledge of Him.  And it really doesn’t fit well with David all of a sudden being comforted with the death of that child.

A book that delves into this topic is Robert Lightner's "Safe in the Arms of Jesus: God's Provision for the Death of Those Who Cannot Believe."