Even the case of the immature and the infirm being included in salvation is anything but airtight. At best a hopeful biblical principle, rather than an explicit statement, can be derived for their salvation. As I see it, three possible grounds exist for inclusivism concerning the immature and infirm: (1) infants have nothing to repent of and so would be included in Christ's resurrection; (2) the immature and infirm are not able to apprehend creation's witness of God, are not truly able to meet the condition of salvation (faith in Christ), and so would be included in the provision of atonement which was made for everyone; and (3) the children of a believer are included in the the body of Christ unless they decide not to believe.
I do not see how infants could have anything of which to repent. Even though they are born into sin and death, separated from God, they have not sinned personally. They have not even had the opportunity to ignore God's witness in creation, so including them in judgment would seem a travesty of justice. It is a God-given principle that children are not made to pay for the sins of their fathers, so it would appear they must be safe.
We do have some disturbing precedents in scripture, however. I wonder, how many infants died in the Flood, or why infants and children were killed by the invading Israelites under the command of God? It seems evident to me that there are mysteries in understanding how God views the situation of children. I search in vain for that one clear, unequivocal passage of scripture that answers these questions.
To me, inclusivist doctrines purporting to understand what God will do in these instances reflect more what the author would like God to do than they report what God said he would actually do. Given Christ's universal atonement, I see the logic in formulating an exclusion to the condition of faith in Christ for those incapable to express such faith through disability. What I do not see is that clearly demonstrated by scripture.