I absolutely hate false doctrines like the "age of accountability". Nothing destroys a passion for truth and the gospel like inventing your own theology - quite apart from Scripture - and then letting that assure you that some people (in this case the young or mentally retarded) don't really need Jesus yet. But this particular Satanic teaching goes a bit further even than that, essentially denying both the headship of Adam in securing the fallenness of the human race, and the necessary headship of Christ in securing an individual's salvation from that curse. Make no mistake, the "doctrine" of the age of accountability is a purely human invention - something to keep parents and preachers from having to trust in the goodness and wisdom of God when a child dies. Instead of trusting that God is in control and that He knows best how to give good gifts to His children (including the trauma of losing a child), we'd rather just spout vaporous and God-demeaning platitudes like, "God just needed him/her more that you did right now" or "Jesus is rocking him/her to sleep at this very moment." Such sentiments are found nowhere in the Bible.
But from time to time, you might hear someone quote a verse like Isaiah 7:16 in support of the "age of accountability". It says, "For before the boy knows how to refuse the evil and choose the good, the land whose two kings you dread will be deserted." We can infer from this verse that there comes a time in a young person's life when he or she can start to differentiate between good and evil. So are they not accountable for their actions before this time? Are they "innocent"?
Let's look again at the verse: "before the boy knows how to refuse the evil and choose the good". According to the context of this verse (verse 15), this most has to do with his taste for certain foods! But let's give those who like to use this verse as a proof for the age of accountability the benefit of the doubt and say that it deals with moral good and evil instead. If that is the case, then what would verse 16 teach? Actually, what we see clearly is that before this time when the boy is able to make the distinction, he is apparently always choosing evil, since we're waiting for him to be able to refuse the evil and choose the good. That doesn't sound very "innocent" at all. That sounds like exactly what we would expect of a human creature tainted by the fallen nature of Adam and born into sinful wickedness according to the Scripture (Psalm 58:3).
So what if a child dies, then, before he/she can accept Christ? Without a doctrine of an age of accountability, what do we do with those who haven't yet had the ability to choose? That very question shows where our thinking has gone wrong. We live in an age that is so man-centered in all of its thought that we elevate human choice to the most important place in any system of theology. What does a person's ability to choose Christ have to do with God's standard of righteousness? God condemns people to hell because of sin, not because they didn't choose Christ (although a rejection of Christ is another sin). Sin and death really did enter the world through one man (Romans 5:12), and it does not wait until some arbitrary age to start showing up. It's there from the start. This is why we all stand condemned from conception. It's why David can say in Psalm 51:5, "Behold, I was brought forth in iniquity, and in sin did my mother conceive me." It's why God can command His people to wipe out even the infants and children of the Canaanites in judgment as they conquer the land.
So is there any hope, then, for a parent grieving over the loss of a child? Absolutely! But that hope is in God's goodness and faithfulness, not in some empty false teaching.
We have a good, perfect, loving, merciful Father who always takes care of us and gives us exactly what we need. The death of a child does not come at Him as a surprise. All things are a part of His plan, and He promises believers that He will work all things for good for those who love Him and are called according to His purpose (Romans 8:28). That means that He had a reason for bringing such pain into the life of a family, and, knowing how good our God is, we know it must be a good reason.
And just as God doesn't have to wait for a person to be old enough to choose before He can condemn that person for his or her sin, He also doesn't have to wait to save them, either. Look at John the Baptist. He was full of the Holy Spirit from the womb (Luke 1:15)! God can do whatever He pleases (Psalm 135:6), so put your trust in Him and His Word when things are difficult, and not some man-made idea.