Jesus already told us that little children are within the Kingdom of God (since they cannot respond to the Gospel). The same would apply to anyone mentally incapable of understanding the Gospel. Christ died for the sins of the whole world, and even John Calvin had to admit that, and stated it in His commentaries. But then he created his own warped theology knowingly.
Okay so you're saying that babies are in the kingdom of heaven because they're babies, but, because they age, they are somehow removed from it? At what age does the Bible instruct that occurs? Knowing that would be a critically important piece of information to have should one's salvation be dependent upon it, wouldn't it? Same with the infirm - while infirm they're in it, but if they improve, they're taken away from it? So then, your belief is that one's salvation depends upon their condition or state at any given point in time? Is that how you see it?
By the above, we can see the problem that arises for those who believe as you do -- those who believe the recipients of salvation must do something to get it. It becomes like a too small bed sheet: pull it one way and the feet are exposed, pull another and the chest is. The same salvation principals must govern everyone equally regardless of a person's particular attributes or situation. Too many exceptions and carve-outs exist for your way to have truly come from God. What makes God's salvation plan so marvelous and wonderful is that He alone gives it to whomever He so chooses to give it, regardless of any external or personal factors. Only God could do that.
By the way, Christ died for the world to come, not this current world. This world is to be destroyed by God not saved.
Regarding Calvin's statement, personally, I don't know, nor do I care what he said, but post it anyway so we can see the whole thing.