The Bible says nothing about an age of accountability.
We do know that God makes decisions prior to any acts of good or evil:
11 for though the twins were not yet born and had not done anything good or bad, so that God's purpose according to His choice would stand, not because of works but because of Him who calls, 12 it was said to her, "THE OLDER WILL SERVE THE YOUNGER." 13 Just as it is written, "JACOB I LOVED, BUT ESAU I HATED." (Rom. 9:11-13, NASB)The gut wrenching question is what happens to infants (including miscarried and aborted babies), toddlers, and the mentally handicapped who never have an opportunity to repent.
From the point of view of Calvinism, there is a normative progression for all believers as described by Paul:
and these whom He predestined, He also called; and these whom He called, He also justified; and these whom He justified, He also glorified. (Rom. 8:30, NASB)Other verses (Rom. 2:4, 2 Cor. 7:9-10, 2 Tim 2:25) link repentance to salvation. Furthermore, repentance is itself is described as a gift from God. It is not a human work, although in appearance it manifests itself as such.
9 I now rejoice, not that you were made sorrowful, but that you were made sorrowful to the point of repentance; for you were made sorrowful according to the will of God, so that you might not suffer loss in anything through us. 10 For the sorrow that is according to the will of God produces a repentance without regret, leading to salvation, but the sorrow of the world produces death. (2 Cor. 7:9-10, NASB)If you must repent to be saved (and you must), then no biblically based system of theology is as encouraging to parents who have lost children as Reformed theology. For it requires nothing of man, no assent, and no self-righteous desire to repent: all are free gifts of God’s grace. There is nothing I did on my own as an adult in order to be saved—God can certainly offer the same gifts to a child from the instant of conception.
Children who are taken into heaven are not innocent, but recipients of God’s grace and mercy.
That is the extremely good news. You don’t have to worry that the child did not have a chance to repent, was not baptized, etc. Salvation is entirely from God. Thankfully, we contribute nothing.
Unfortunately, the Bible does not offer any assurances that all children who die receive the gift. Most of us make an appeal to God’s mercy in finding hope that such children were elect and did receive grace although too young to display any outward signs. Some Reformed denominations hold that children of believers are given special dispensation of grace due to the covenant God made with Abraham and his descendants.
One of the great encouragements comes from King David, whose child born of his adultery with Bathsheba died. David wrote, of the time just after the child’s death:
22 He said, "While the child was still alive, I fasted and wept; for I said, 'Who knows, the LORD may be gracious to me, that the child may live.' 23 "But now he has died; why should I fast? Can I bring him back again? I will go to him, but he will not return to me." (2 Sam 12:22-23, NASB)There is no age of accountability. That would mean our salvation is in our hands, and that would be really bad news. Salvation is a free gift of grace that God dispenses to whom He pleases, including, I am sure, infants in the womb. If my wife and I had lost a child through a miscarriage, I would fully expect to see that child in heaven.