Objection 1. It seems that those who have not the use of reason ought not to receive this sacrament. For it is required that man should approach this sacrament with devotion and previous self-examination, according to 1 Corinthians 11:28: "Let a man prove himself, and so let him eat of that bread, and drink of the chalice." But this is not possible for those who are devoid of reason. Therefore this sacrament should not be given to them.
Objection 2. Further, among those who have not the use of reason are the possessed, who are called energumens. But such persons are kept from even beholding this sacrament, according to Dionysius (Eccl. Hier. iii). Therefore this sacrament ought not to be given to those who have not the use of reason.
Objection 3. Further, among those that lack the use of reason are children, the most innocent of all. But this sacrament is not given to children. Therefore much less should it be given to others deprived of the use of reason.
On the contrary, We read in the First Council of Orange, (Canon 13); and the same is to be found in the Decretals (xxvi, 6): "All things that pertain to piety are to be given to the insane": and consequently, since this is the "sacrament of piety," it must be given to them.
I answer that, Men are said to be devoid of reason in two ways. First, when they are feeble-minded, as a man who sees dimly is said not to see: and since such persons can conceive some devotion towards this sacrament, it is not to be denied them.
In another way men are said not to possess fully the use of reason. Either, then, they never had the use of reason, and have remained so from birth; and in that case this sacrament is not to be given to them, because in no way has there been any preceding devotion towards the sacrament: or else, they were not always devoid of reason, and then, if when they formerly had their wits they showed devotion towards this sacrament, it ought to be given to them in the hour of death; unless danger be feared of vomiting or spitting it out. Hence we read in the acts of the Fourth Council of Carthage (Canon 76). and the same is to be found in the Decretals (xxvi, 6): "If a sick man ask to receive the sacrament of Penance; and if, when the priest who has been sent for comes to him, he be so weak as to be unable to speak, or becomes delirious, let them, who heard him ask, bear witness, and let him receive the sacrament of Penance. then if it be thought that he is going to die shortly, let him be reconciled by imposition of hands, and let the Eucharist be placed in his mouth."
Reply to Objection 1. Those lacking the use of reason can have devotion towards the sacrament; actual devotion in some cases, and past in others.
Reply to Objection 2. Dionysius is speaking there of energumens who are not yet baptized, in whom the devil's power is not yet extinct, since it thrives in them through the presence of original sin. But as to baptized persons who are vexed in body by unclean spirits, the same reason holds good of them as of others who are demented. Hence Cassian says (Collat. vii): "We do not remember the most Holy Communion to have ever been denied by our elders to them who are vexed by unclean spirits."
Reply to Objection 3. The same reason holds good of newly born children as of the insane who never have had the use of reason: consequently, the sacred mysteries are not to be given to them. Although certain Greeks do the contrary, because Dionysius says (Eccl. Hier. ii) that Holy Communion is to be given to them who are baptized; not understanding that Dionysius is speaking there of the Baptism of adults. Nor do they suffer any loss of life from the fact of our Lord saying (John 6:54), "Except you eat the flesh of the Son of Man, and drink His blood, you shall not have life in you"; because, as Augustine writes to Boniface (Pseudo-Beda, Comment. in 1 Corinthians 10:17), "then every one of the faithful becomes a partaker," i.e. spiritually, "of the body and blood of the Lord, when he is made a member of Christ's body in Baptism." But when children once begin to have some use of reason so as to be able to conceive some devotion for the sacrament, then it can be given to them.
Source: Summa Theologica (New Advent)