John, Peter, and Paul, for instance, implied that the saving response to the gospel involves a rational choice. True, the apostles did not expressly say that people will be saved only if they repent, believe, and confess.
Christian traditions bring a variety of perspectives to the matter.
Until that age, the child is innocent. Some might extend this doctrine to cover the spiritual state of mentally disabled individuals as well. I am of another mind. I believe that we should, on biblical grounds, tell all parents of mentally disabled children that God loves their children, regrets terribly that they are disabled, and will, when they die, carry them gently into a heavenly life where every person is forever whole. Let me try an analogy.