How does God handle the prayers of Christians who are no longer mentally able, such as the very senile elderly, or mentally ill children who are taught to pray by their parents? What about children in general?
"Because I delivered the poor who cried out, the fatherless and the one who had no helper" (Job 29:12).
God says He watches over those unable to defend themselves: the innocent, the downtrodden, widows, and orphans. "But You have seen, for You observe trouble and grief, to repay it by Your hand. The helpless commits himself to You; You are the helper of the fatherless" (Psalms 10:14). "A father of the fatherless, a defender of widows, is God in His holy habitation" (Psalms 68:5).
Whether someone has not yet gained the mental capability to be an adult, will never gain it, or has lost it the result is still the same. God keeps watch over them whether they are able to pray or not.