Objection 1. It would seem that this sacrament should be given to madmen and imbeciles. For these diseases are full of danger and cause death quickly. Now when there is danger it is the time to apply the remedy. Therefore this sacrament, which was intended as a remedy to human weakness, should be given to such people.
Objection 2. Further, Baptism is a greater sacrament than this. Now Baptism is conferred on mad people as stated above (III, 68, 12). Therefore this sacrament also should be given to them.
On the contrary, This sacrament should be given to none but such as acknowledge it. Now this does not apply to madmen and imbeciles. Therefore it should not be given to them.
I answer that, The devotion of the recipient, the personal merit of the minister, and the general merits of the whole Church, are of great account towards the reception of the effect of this sacrament. This is evident from the fact that the form of this sacrament is pronounced by way of a prayer. Hence it should not be given those who cannot acknowledge it, and especially to madmen and imbeciles, who might dishonor the sacrament by their offensive conduct, unless they have lucid intervals, when they would be capable of acknowledging the sacrament, for then the sacrament should be given to children the same in that state.
Reply to Objection 1. Although such people are sometimes in danger of death; yet the remedy cannot be applied to them, on account of their lack of devotion. Hence it should not be given to them.
Reply to Objection 2. Baptism does not require a movement of the free-will, because it is given chiefly as a remedy for original sin, which, in us, is not taken away by a movement of the free-will. On the other hand this sacrament requires a movement of the free-will; wherefore the comparison fails. Moreover Baptism is a necessary sacrament, while Extreme Unction is not.
Source: Summa Theologica (New Advent)