What does the Bible say about salvation of those mentally unable to comprehend God's Word, or those that are too young to mentally understand?

Believe me when I say that I’ve thought a lot about this. Does God hold someone guilty who lacks the ability to accept, or reject, eternal life in Jesus Christ? No, He doesn’t.

Is there a verse about this issue in the Bible? No. But the principle is there. While the Bible doesn’t address the salvation of someone who is MR or autistic specifically, we do clearly see salvation graciously given to that other demographic in the question: the too-young. Babies.

David and Bathsheba’s baby died in 2 Samuel 12. (Go read the story!) David mourned the child’s illness, and worshiped God after the child died. What? Why worship? Because faith in God isn’t based on whether or not I like everything He’s doing. I worship Him because He deserves it and I trust Him. In this account, David knew that the baby wouldn’t be coming back to David BUT instead David would someday go to where the baby was - heaven.

I believe the same grace applied to this baby is applies to all babies… and all others who are unable to either accept or reject Jesus Christ.