An answer to the question is hinted at in a few places in the Scriptures. In John 15 Jesus speaks of the religious leaders who had rejected Him stating that their guilt was increased by their knowledge:
Joh 15:20 Remember the word that I said unto you, The servant is not greater than his lord. If they have persecuted me, they will also persecute you; if they have kept my saying, they will keep yours also.
Joh 15:21 But all these things will they do unto you for my name's sake, because they know not him that sent me.
Joh 15:22 If I had not come and spoken unto them, they had not had sin: but now they have no cloke for their sin.
Joh 15:23 He that hateth me hateth my Father also.
Joh 15:24 If I had not done among them the works which none other man did, they had not had sin: but now have they both seen and hated both me and my Father.
This suggests that men are accountable to God for what they are aware of. You also have Paul's discourse in Athens:
Act 17:29 Forasmuch then as we are the offspring of God, we ought not to think that the Godhead is like unto gold, or silver, or stone, graven by art and man's device.
Act 17:30 And the times of this ignorance God winked at; but now commandeth all men every where to repent:
Act 17:31 Because he hath appointed a day, in the which he will judge the world in righteousness by that man whom he hath ordained; whereof he hath given assurance unto all men, in that he hath raised him from the dead.
There are at least two things that are universal, even without the Scriptures, which Paul mentions in Romans. It should be noted that his main point was to show that people had rejected God, but nonetheless it speaks of the knowledge of God that all have. The first is the creation:
Rom 1:19 Because that which may be known of God is manifest in them; for God hath shewed it unto them.
Rom 1:20 For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead; so that they are without excuse:
The second is the conscience:
Rom 2:11 For there is no respect of persons with God.
Rom 2:12 For as many as have sinned without law shall also perish without law: and as many as have sinned in the law shall be judged by the law;
Rom 2:13 (For not the hearers of the law are just before God, but the doers of the law shall be justified.
Rom 2:14 For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves:
Rom 2:15 Which shew the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another)
Rom 2:16 In the day when God shall judge the secrets of men by Jesus Christ according to my gospel.
There are also texts such as Acts 10 that suggest that those who are seeking God may receive at times supernatural revelations. THis is not to say it always happens, but that it does happen at times.
The upshot to me of the biblical evidence is that while everyone is not familiar with the name of Christ He has still bought the salvation of all who are willing. While all do not understand all the details it is a near universal human realization that we fall short of moral behavior. Therefore those who turn to God in love, seeking for forgiveness, can be saved.
God is not willing that any should perish, so I assume that He extends that possibility to all:
2Pe 3:9 The Lord is not slack concerning his promise, as some men count slackness; but is longsuffering to us-ward, not willing that any should perish, but that all should come to repentance.
As to those who do not have mental capacity, I am not immediately recalling a text that directly addresses the point. I simply trust in the grace of God and His desire that all men be saved.