If someone at birth is unable to comprehend anything at all or consider special ed will they be able to go to Heaven or Hell, considering they are unable to learn anything?

It has always been my understanding that innocent children, who have not yet sinned, will be in heaven.  Innocent people have no reason to fear judgment from God.  This is clear.  Jesus said concerning children, in Matthew 18:10, “I tell you that their angels in heaven always see the face of my Father in heaven.”  There are Christian groups who teach a false doctrine known as “Original Sin.”  This false and unbiblical teaching is that babies are born having inherited “original sin” from the sin of Adam, and are therefore guilty of sin at birth. This doctrine is not supported by any Scripture that I know of.  Instead, it was invented as a justification for the false practice of infant baptism in the fourth century.  Be assured, babies are safe in the arms of God and will not go to Hell.

The second part of your question is a bit more challenging.  I am convinced that those who are so mentally challenged as to be, in essence, children are, like children, pure and incapable of sin.  An extremely mentally limited person is in almost every way mentally a child.  I am convinced that, as intellectual children, they are in the same spiritual situation as actual children.

The reason this is a different situation is that I do not have a biblical passage specifically dealing with this situation. Also, making it more challenging is the fact that mental ability exists on a continuous range.  How mentally limited must one be to be innocent before God?  Is a person with an IQ of 50 child-like and therefore innocent of sin?  What about a person with an IQ of 80?  The answer is that we cannot make such a distinction.  God is the ultimate judge of such a thing, not us.  We cannot remove all ambiguity on this front, and will have to live with at least some level of doubt.  My suggestion is that if a person is open to the teaching of Christ and if there is any doubt about whether they are sufficiently challenged to be innocent, we should take the better-safe-than-sorry approach and study with such a person and, if they say they appear to be repentant and understanding of the most basic gospel story, then we should baptize them.

I hope this slightly ambiguous answer is helpful.