I honestly don’t believe many low functioning people require the standard “forgiveness.” They are as innocent as babies as far as I can tell, more so in some cases...they aren’t restricted by infant bodies.
Just my thought. It won’t hurt my theology either way.
IMO our theology "predicts" the type of place we make for the mentally disabled among us. As a Calvinist, I believe in Total Depravity, which is to say that every single part of a man has been touched by sin. We sin because we are sinners; if we say that someone does not sin, we are saying that they are not a sinner. David says that he had been a sinner even in his mother's womb (Psalm 51:5). IMO there are no "innocents" of any age that have never sinned.
There is no corner of creation that has not fallen. To believe this would mean some have no need of the mercy and forgiveness of Christ. Or, as I argued in my previous post, that to believe so is to pronounce that some have a need but due to their infirmities, they do not need to be told of / cannot respond to the grace and mercy and healing of Christ's blood.
God Bless you!