This question was asked in the context of the Amalekites that were devoted for destruction by God in 1 Samuel 15:3. Are the Amalekite infants and children who were slaughtered in heaven since they were younger than the age of accountability?
To answer this question, I need to change the question. I do not believe in an age of accountability. I do not see where the Bible teaches it.
The whole idea is a holdover from Judaism. A person was believed to be innocent until they could discern moral right from moral wrong. That is why the Bar/Bat Mitzvah would happen. It literally means “son/daughter of the commandment.” It was the age that a Jew would take responsibility for their own actions before God. In the Middle Ages, the Roman Catholic Church added the sacrament of Confirmation to be pretty much the same thing. Only in the Roman Catholic Church, they believe that baptism washes away original sin, and it is only individual sins that are the basis for judgment. When someone is “confirmed,” they are confirming their baptism and taking responsibility for their actions. Neither of these systems (Judaism or Roman Catholicism) believe that grace alone saves; our actions, at least in part, determine our standing with God.
In modern Protestantism, the whole “age of accountability” thing is an Arminian doctrine. They believe that you have to “accept” Jesus and His salvation in order to be saved. You have to do something. So until someone is mentally able to understand the Gospel and what they are accepting, they cannot be held responsible. They will often appeal to David’s comments about his dead son (see 2 Sam 12:23) and the places the Bible speaks of children as not knowing good from evil (see Deut 1:39, Isa 7:15-16) as proof of the doctrine. However, David could just as easily be speaking of death. His child can not come back to the world of the living, but David will eventually go to the place of the dead. In the case of Deuteronomy 1:39, God is speaking about one particular sin: the unbelief of that whole generation (save for Joshua and Caleb). God is taking from them what they were promised if they obeyed and is giving it to their children. Isaiah 7:15-16 is referring to a child not knowing morally right from morally wrong, but that speaks about knowledge, not about guilt before God.
The question really is this: is knowingly committing morally evil acts the basis for reprobation?
While the reprobate will be judged for what they do, this is not why they are excluded from salvation. Revelation 20:11-15 is a popular passage for those who claim it is our moral actions that result in reprobation. But while the reprobate are here said to be judged for their actions, the reason they are thrown into the lake of fire is that their names are not written in the book of life (Rev 20:15). Yes, we are responsible for our actions, but it is our sin – not our sins – that condemn us. It is our very nature as those in Adam – not our actions – that make us guilty before God (Ps 51:5). We are all by nature condemned, and it is only God’s grace that saves us (Rom 5:12-21, Eph 2:1-10). This being the case, our actions at no point determine our standing with God, whether an infant or an adult. In addition, on this side of the cross, discerning good from evil is a spiritual matter; not a matter of physical maturity, but spiritual (Heb 5:11-14, 1 Cor 3:1-3).
Now, to answer the heart of question, if we take the witness of the entire Bible and what it says about the nature of God, I would say that it is not out of the realm of possibility that God in His grace only allows the elect to die in infancy. The Bible doesn’t say that, but the character of God certainly doesn’t exclude it, and may even lead us to that conclusion. However, we need to balance that with passages like Romans 9, where God is said to prepare vessels of wrath and vessels of mercy from the womb (and even before that). In the case of the Amalekites (or any of the people devoted to destruction), they were sentenced to death because of their sin. The physical death is a type or picture of spiritual death for those outside of God’s chosen people. But in the Old Testament, God punished people (including Israelites – see Num 16:25-33) along with their children when they sinned. Does this mean the children were guilty of sin, too? I don’t think that is a necessary conclusion.
So there are two questions here. First, does the Bible teach an age of accountability? No. Second, do infants and children who die go to heaven? The Bible doesn’t give a definitive answer, but it reveals enough about the character of God to give us a reasonable hope that the answer is yes.