In Christian theology it is believed that until a child reaches the age of maturity or accountability, they are covered by the grace of God as a believer. God gives us creation and our conscience to know that God exists and what is right and wrong, but there comes a point where a child is developed enough mentally and physically and they become responsible for their choice of coming to God or following after the world. This age is believed to be around 12 years old, but each child develops differently based on their environment and what knowledge they have from parents and society. And God can look into the heart to know when the time has come.
Romans 1:19–21
[19] For what can be known about God is plain to them, because God has shown it to them. [20] For his invisible attributes, namely, his eternal power and divine nature, have been clearly perceived, ever since the creation of the world, in the things that have been made. So they are without excuse. [21] For although they knew God, they did not honor him as God or give thanks to him, but they became futile in their thinking, and their foolish hearts were darkened. (ESV)
John MacArthur has a great explanation below and also wrote a book on the subject for parents that lost children. But following this logic, when God extracts His true church from the world included would be the children that haven’t reached the age of accountability, who are covered by grace as is the body of believers. And God has told us about releasing His wrath on those that reject Him in the Great Tribulation, so how could He leave these children behind to go through that horrendous period or be subject to the evil done by those left on the earth? And we see the unbridled evil that is being unleashed already, and how fast will it escalate when God’s elect are removed from the earth. Consequently, we can take this recounting of David’s child dying and his confidence that he would go to him as evidence of God’s grace covering children.
2 Samuel 12:18–23
[18] On the seventh day the child died. And the servants of David were afraid to tell him that the child was dead, for they said, “Behold, while the child was yet alive, we spoke to him, and he did not listen to us. How then can we say to him the child is dead? He may do himself some harm.” [19] But when David saw that his servants were whispering together, David understood that the child was dead. And David said to his servants, “Is the child dead?” They said, “He is dead.” [20] Then David arose from the earth and washed and anointed himself and changed his clothes. And he went into the house of the LORD and worshiped. He then went to his own house. And when he asked, they set food before him, and he ate. [21] Then his servants said to him, “What is this thing that you have done? You fasted and wept for the child while he was alive; but when the child died, you arose and ate food.” [22] He said, “While the child was still alive, I fasted and wept, for I said, ‘Who knows whether the LORD will be gracious to me, that the child may live?’ [23] But now he is dead. Why should I fast? Can I bring him back again? I shall go to him, but he will not return to me.” (ESV)
https://www.gty.org/library/Articles/A264/The-Age-of-Accountability
The following article is adapted from John MacArthur’s response during a Q&A session at Grace Church. Much of what John says here has been expanded in his book, Safe in the Arms of God .
What is the Scriptural basis for an “age of accountability” regarding a child’s salvation?
I think the best way to answer that is to say this: There is no “age of accountability” identified in Scripture, as such. There is nothing in the Bible that says, “Here is the age and from here on you are responsible!” I think the reason for that is because children mature at different paces. That would be true from culture to culture, and from age to age in history.
So the Lord in His wisdom didn’t identify a specific moment. God knows when each soul is accountable. God knows when real rejection has taken place; when the love of sin exists in the heart. When enmity with God is conscious and willful. God alone knows when that occurs.
The Jews had identified about the age of twelve, and that was when Jesus was taken by His parents to Jerusalem for the Passover and the Feast (Luke 2:41-42), and there He was in the temple questioning the doctors. You have a good illustration there, and Jesus was asking profound questions at that point. This then seems to be the age when those kinds of questions begin to be personal in the heart of a child.
So I have always felt that somewhere around age twelve, the transition from childhood to adulthood takes place. It’s probably not totally disassociated from puberty, where there is a consciousness of one’s own impulses, feelings, drives, desires, and therefore sinful attitudes and passions, and whatever else starts to emerge.
With this in mind, I believe that it is absolutely essential, all along the way with children, that every time they desire to make a commitment to Jesus Christ, at whatever age, you (as someone giving spiritual oversight to them) encourage them to do that. Because you don’t know, we can’t know, when their desire is indicative of genuine saving faith. When a young child says, “I want to invite Christ into my life,” then you need to encourage them to do that. Every one of those, I see as a step towards God. At what point that becomes saving faith—only God knows for certain.
But, I also believe, that up until that point of real saving faith, God in His mercy, would save that child, should that child die. I have been doing some study on that very issue, because when I was at a conference recently, and that question was asked of a panel of very astute theologians—no one gave an adequate answer. And I thought, “How can we have theologians who don’t know the answer to that question? What about the children before the age of accountability, when they die, do they go to heaven?” I think the answer is “yes,” and I think it is a strong “YES,” based upon the confidence of David who said, when his little baby died: “He cannot come to me, but I shall go to him.” And David knew where he was going; David knew he was going to heaven—he knew that. There wasn’t any question in his mind about that.
So when he said, “I shall go to him,” in those words was the anticipation and the joyful hope of reunion. Now, some people have said, “Well, all he meant was, ‘I am going to be buried next to him.'” There wouldn’t be any reason to say, “He can’t come to me, but, oh I’m so glad I am going to be buried next to him!” There would be no joy in that; that wouldn’t satisfy anything. So I think at that point, he was expressing the confidence that he was going to heaven; he knew that was where he would find his son, who had died before the age of accountability.
Another interesting thing that occurs numerous times in the Old Testament, is that children (including those who die) are referred to as “innocent.” The Hebrew word that is used for “innocent” is used numerous times in the Old Testament to refer to “not being guilty”—literally, “being taken to court and found ‘not guilty.'” In fact, the Old Testament refers to the babies that were passed through the fire to Moloch [false god] as the “innocents,” so I believe that God, prior to the “age of accountability” treats them as “innocent.” It doesn’t mean that they are not fallen; it doesn’t mean that they are not sinful—it does mean that God mercifully treats them as “innocent” in spite of that, and He has to exercise grace to do that, just as He exercises grace to save those who believe.
In summary, the “age of accountability” is not clearly identified in Scripture. I think it’s up to parents; every time a child wants to respond and open the heart to Christ—you need to encourage that, all the way along, until they come to that point where it is genuine, and the Lord knows that even if you don’t.