Death is the most devastating effect of the Fall, and the death of a baby—the most innocent of humanity—is a trophy kill for Satan. Likewise, Satan delights in birth defects, like permanent mental disabilities, because it’s his attempt to tarnish the gem of God’s creation. In these cases death can come before the person is capable of making a decision for Jesus (Rom 10:9-10), who is the only way into heaven (Jn 14:6), and so it is natural to question the eternal destiny of such individuals.
The scriptures do not explicitly affirm this, but there are several passages that strongly imply babies and the mentally handicapped go to heaven when they die.
First, while it is true babies and the mentally handicapped don't have the ability to accept Jesus, it is also true they don't have the ability to reject Jesus, which is an important consideration because the scriptures teach judgment is the result of the rejection of God’s revelation (Lk 10:16; Jn 12:48; 1 Thess 4:8).
Paul discusses this deeper in Romans 1:20, where he says men are without excuse of judgment because of the general revelation of God, which is the evidence of God in creation. John Piper says “mankind would seem to have an excuse if they had not seen clearly in nature what God is like,” and so, because babies and the mentally handicapped are incapable of processing the general revelation of God, it seems they would fall into the category of having an excuse.
This is buttressed by texts that assert babies, in particular, don’t know the difference between good and evil. Deuteronomy 1:39 reads, “… your little ones … who this day have no knowledge of good or evil.”
While babies and the mentally handicapped aren’t capable of rejecting Jesus, this doesn’t mean they aren’t participants in the Fall, as if they are some kind of neutral bystander. David says, “I was brought forth in iniquity, and in sin my mother conceived me” (Ps 51:5). Paul writes, “… sin entered the world, and death through sin, and so death spread to all men” (Rom 5:12). The question isn’t whether such individuals are subjected to the Fall and its penalties, but how God deals with them.
The Bible shows God as “merciful and gracious, slow to anger and abundant in lovingkindness” (Ps 86:15). He is “righteous in all his ways and kind in all his deeds” (Ps 145:17). The most merciful and gracious act of all was God sending Jesus to die for the sins of the wicked world. John identifies Jesus as the propitiation for the sins of the "whole world," which includes babies and the mentally handicapped, and it’s possible that God, in his mercy, grace, lovingkindness and righteousness, applies the sacrifice of Christ to those who are incapable of rejecting or accepting him (1 Jn 2:2).
God's heart for children is easily observed through Jesus, who called a child to himself to use as an example for the disciples who were arguing with one another about who among them is the greatest in the kingdom: “Truly I say to you," Jesus said, "unless you are converted and become like children, you will not enter the kingdom of heaven. Whoever then humbles himself as this child, he is the greatest in the kingdom of heaven” (Mt 18:2-4). Jesus later blessed children, saying, “Let the children alone, and do not hinder them from coming to Me; for the kingdom of heaven belongs to such as these” (Mt 19:14-15).
We often think of babies and children as if they are the incompetent ones, lucky to edge their way into heaven, but Jesus describes them as if they are the ones that epitomize the character of heaven's best citizens. I tend to wonder if babies in heaven ask God, "Do adults and the mentally able go to heaven when they die?"
While these verses strongly imply that babies and the mentally handicapped go to heaven when they die, there is an event in Scripture which is about as close to an explicit answer as possible. In 2 Samuel 12:23 David, upon losing his baby in death, tells his servants he will “go to” his child one day, indicating he would be reunited with his child in the Old Testament epoch of heaven.
The scriptures strongly imply, via the grace of God, that babies and mentally handicapped individuals enter heaven upon death. The implications of this are comforting for those to whom this is a very personal question. This means every person who never reached a point of accountability is right now in the bosom of God himself, which includes every aborted baby, miscarried baby, and person who was born with mental disabilities that kept him from accepting Jesus before death.
If there is an example of grace in salvation, it's that babies and the mentally handicapped go to heaven when they die (Eph 2:8-10). The Fall is strong, but God is stronger. The grace of God in appropriating his forgiveness of sin to such individuals illustrates this at the most profound level.