While of course, it is in no man's authority to make that judgment, being "the big one", the one where we discern who goes to Heaven and who goes to Hell, we must rely on God's omniscience, his infinite wisdom to at least remotely grasp some sensible apologetics for this inevitable question. If we believe that God is indeed a good God, then we must also believe that he possesses a perfect and holy judgment parallel to that goodness.
Now first of all, I want to note that oftentimes, leaning on our own understanding is ultimately what keeps us worrying and despairing. One is commanded not to lean on his own understanding for such reasons as these (Proverbs 3:5); yet it is ironic that many tend to do this for some faux sense of security, especially for the skeptic who doubts the goodness of God thus fallaciously doubting the overall existence of God.
God abhors sin because it brought and still brings suffering to all men and women, including tiny infants in large hospital beds who have never done a thing wrong in their short periods on earth. His anti-sinful nature is a direct reflection of his perfect holiness; his perfect holiness is a direct reflection of his omniscient judgment; and his omniscient judgment is a friend rather than an enemy of faithful men and all who trust in him: if you hate the suffering of the innocent, then, as God does, you must also hate sin, the origin of that suffering.
But to answer the initial question, one might safely assert that God knows the mental and spiritual capacity for each individual and what truths each individual is capable of understanding during the time that he or she lives on earth. I do not think that God has a universal age limit where he must judge whether someone will go to Heaven or Hell. Instead, he takes everything into account for each individual: brain capacity, spiritual awareness, genetic bias, opportunities to understand the truth of having faith in Christ before death, and the extent of cultural influence in conflict with one's God-given conscience. Furthermore we can take into account the idea that God also takes into account, as another tool in his omniscient judgments, his all-knowing foreknowledge regarding who each individual would have become if he had lived a longer or mentally stable life. One might never know that a baby who has passed away could have been saved from a long life of misery or pain or confusion or unfairness or anger, hence receiving what, in reality, seems like a quick, simple flight to Heaven.
God judges men from the inside out; men judge men from the outside in. Perhaps to God, an extreme mental patient is doing quite well in going a month without murder, for he fought his chemical imbalance and succeeded; oppositely, perhaps the healthy, able and stable man who has never murdered in his life yet went a lifetime consciously, willingly never loving anyone but himself may then be subject to harsher judgment than the extreme mental patient. It might be so that God will stand for the weak and question the strong.
Thank God that he knows these details for each individual whether the circumstances are healthy, sick, young, old, wise, foolish, pressured, or excluded and personality-wise, genetic-wise, and culture-wise. Although there may be no way for us to discern them by mere mortal perception, God always knows his babies and his psychopaths. It is okay, ultimately, therefore we can release such worries and instead rely on faith in his good and everlasting knowledge, grace, love, and mercy - which is where Christ comes in. With the One, there is no circumstance or burden which he cannot cover and one cannot sequentially bear. The case of where infants, children, and the mentally ill, who cannot comprehend and therefore accept the love of Christ,
go after death is yet another one of the countless reasons Christ died
for mankind. Above all human reasoning, it is not by our own works, but by his works, that we are saved. Therefore, in short, it is safe to say that the grace of Christ, like it covers any sinful yet conscious adult, also covers the innocent infant and the mentally-impaired psychopath.