So many sufferers of cognitive dissonance ..... so little time!!
Do souls who die as infants need a savior?
Do mentally incapacitated individuals need a savior?
Neither are capable of sinning. Are they left out in the cold?
Yes.
Do mentally incapacitated individuals need a savior?
Yes again.
Neither are capable of sinning. Are they left out in the cold?
No, they are not left in the cold, and yes, they are capable of sinning. All humans are born with a sin nature, born sinful. If you hold any other belief, then you start to come up with a sort of "age of responsiblity" sort of problem; "Well, children don't know any better so they aren't sinful until they're, say, five." Unfortunately, this is both Scripturally and logically unsupported.
Besides, if we say that you have to be responsible before you can be accounted sinfully, you start getting really nasty ethical questions that don't pop up if you simply hold that everyone, from the oldest man or woman down to the newly-concieved child, needs a Savior.