Since I have been asked this several times, I thought I would repost a discussion I participated in on Rapture Forums last year (March 2022) on the subject of whether children go up in the Rapture. Here was my updated response.
Although it's not clearly spelled out in Scripture, I believe the issue itself is very straightforward:
1. God created man with the capacity to believe and choose right over wrong. This is what distinguishes us from every other living creature
2. Man physically reaches full mental cognizance when the brain is fully developed (age 20 and upward). God consistently uses the age of 20 in the OT as the dividing mark between accountable and unaccountable (eternally speaking). (Ex. 30:11-16, Num. 32:11) There is no age given in the NT (I believe God defaults to save any who come to Him whatever age they may be)
Then the Lord spoke to Moses, saying: 12 “When you take the census of the children of Israel for their number, then every man shall give a[a] ransom for himself to the Lord, when you number them, that there may be no plague among them when you number them. 13 This is what everyone among those who are numbered shall give: half a shekel according to the shekel of the sanctuary (a shekel is twenty gerahs). The half-shekel shall be an offering to the Lord. 14 Everyone included among those who are numbered, from twenty years old and above, shall give an offering to the Lord.15 The rich shall not give more and the poor shall not give less than half a shekel, when you give an offering to the Lord, to make atonement for yourselves. 16 And you shall take the atonement money of the children of Israel, and shall appoint it for the service of the tabernacle of meeting, that it may be a memorial for the children of Israel before the Lord, to make atonement for yourselves.” Exodus 30:11-16
‘Surely none of the men who came up from Egypt, from twenty years old and above, shall see the land of which I swore to Abraham, Isaac, and Jacob, because they have not wholly followed Me, Numbers 32:11
Two points: Those under the age of 20 were not permitted to pay a ransom (for atonement). Exodus doesn't specify as to why, or whether or not those 19-year-olds were mature enough, or good or bad. It just sets the standard at a particular age of division- 20 years of age and above. Numbers 32:11 also list those who were not permitted to enter into the Promised Land (those 20 and up). Said nothing about the worthiness or maturity of those who were 19 and below. Again, God sets that as the age of division between being morally culpable and those who are not.
3. God then sets the conditions for salvation based on our ability to believe in Him. Before Christ came, it was man's faith in the living God. After Christ came, our faith must be directed at God-manifested in the flesh, Jesus the Christ, and His finished work at the Cross.
4. Modern biology has come to study and learn much about the human brain, and one of those conclusions is that the last part of our brain that gets fully developed is our pre-frontal cortex, which governs what we would call higher critical thinking. Actually, our brain sort of develops all at once, but we see generally see it develop from the back (which governs motor skills) to the front (critical thinking). Critical thinking is developed by our experiences, education, imagination, emotion, etc. These things obviously take time. Setting biology aside for a moment, we need to make sure we don't confuse mental cognitive skills with mental/ emotional maturity. I (being a typical kid) can remember being mentally cognitive as a sixteen-year-old, but I was far from mature and I had very little understanding of consequences- especially consequences with long-term (or eternal) ramifications.
1. God could have created mankind to reach full mental cognition at a much earlier age (say age 5 or 13), but He didn't. He doesn't explain why either.
2. Does mental cognition equal maturity? Yes and no. We all mature at different rates, but maturity does not supersede the age standard God set forth in Scripture. IOW, you can be immature over the age of 20, but at that point, it's on your head because you have been given the tools and you chose at that point what to do with them. (We do the same with adulthood being at 18, and breaking the law at that point carries far more severe consequences than if you were say, 17 years old).
3. The mentally incapacitated (those with varying forms of Downe Syndrom), and children up to the age of 20 (the only consistent biblical age we have to work with) that have not yet reached that full capacity by the Rapture, will be taken up and afforded the same level of mercy as those who died before 20 receive, and have received throughout all of time.
4. After the Dispensation of the Church ends, mankind is thrust back into the conclusion of a post-Christ dispensation of Law (the conclusion of it rather). Those who enter into it must be afforded the opportunity to either receive Christ or reject Him with full mental cognition intact, or else God's manner of justice would not be perfect in sentencing them to eternal condemnation.
1. Now, if we argue that God grants unwarranted mercy on those who, for instance, such as babies and the mentally incapacitated, on what basis then do we make that argument? Well, we could say that according to God's own word, He is longsuffering that none should perish. (Ex. 34:6, Ps. 86:15, 2 Peter 3:9) How then can some argue that God should send babies (who lack the ability to believe) as well as those who are mentally incapacitated (who also lack the ability to believe) to eternal hell-fire for something they had no way of changing?
Does that sound like God? Does that sound like any reference to God in Scripture anywhere?
2. Let's make the distinction, as this issue will come up, that God allows children and the mentally incapacitated to suffer all manner of evils and death in this life. People can point to the Flood or Sodom and Gomorrah. That is true. Children and the mentally incapacitated have suffered all manners of evils in this world. That is simply part of living in a fallen world. But the first death is of no divine significance when compared to the second death. That's not to say God delights in their tragic endings, heavens no. What I mean is, while people (and animals) might suffer in this life, God will indulge them gloriously for all eternity because of that injustice. If Ephesians 2:4-7 is any indication, we can't really begin to understand how wonderful this will all be for us.
When we speak of eternity, well, that is God's realm, and in His realm, He is just and merciful, and all of His attributes are the very epitome of perfection.
Fairness: For example, would we call a human judge here on earth, who takes a child guilty of (pick a crime) and sentences him/her to life in federal prison- a just or cruel judge?
Has it happened? Sure. And we would consider this judge cruel for condemning a child who lacks not only the ability to clearly distinguish, right from wrong but also the ability to understand the consequences of right and wrong.
If we (fallen humans) can consider that judge cruel, and we build up laws to guard against things like that because we recognize the unfairness of it, how much more does God understand perfectly the issues of culpability and innocence?
3. None of us deserve salvation. If God showed us how sinful and wicked we truly are, we would crumble into an unrecoverable tailspin of grief and misery, unable to ever be useful to Him or His purposes. Thankfully, God provides that grace and mercy to us through His Son's perfect sacrifice. If God can afford us wicked people a means to salvation, how much more would HE for those who can't understand eternal ramifications? (Mark 10:14)
4. If we look at the history of this throughout the Church Age, I see some major issues of corruption that spring from the death and innocence of children. We could look at the Roman Catholic's power over family through infant baptism and purgatory. We could look at Calvinism and its corrupted version of predestination. Both of these are perversions of what the Bible says, and they both seek to place religion/religious piety over Jesus' free gift of salvation and God's longsuffering nature.
1. God created mankind to be unique in our ability/capacity to both believe and choose life (God) over death (hell and Satan)
2. God designed mankind to physically arrive at that mental/cognitive destination (biologically) at the age of 20 and above. We see this demarcation line clearly throughout the first five books of the bible.
3. Can a child receive Christ, even though that child has zero understanding of eternal consequences? Absolutely. God defaults to save any who calls upon Him and He will refuse none.
4. Can a thief on a cross receive Christ, having done absolutely nothing to warrant it? Absolutely.
5. Will the Rapture be a once in mankind event that violently separates two dispensations, unleashing hell on earth? Yes
6. Would a just-God, allow a child who cannot make that eternal decision, to enter into a time when a mark will be imposed upon all, which then condemns the bearer of said mark to eternity in hell?
Don't we make this same argument for why Christians can't be in the 70th Week?