Objection 1. It would seem that children would have had perfect use of reason at birth. For that children have not perfect use of reason in our present state, is due to the soul being weighed down by the body; which was not the case in paradise, because, as it is written, "The corruptible body is a load upon the soul" (Wisdom 9:15). Therefore, before sin and the corruption which resulted therefrom, children would have had the perfect use of reason at birth.
Objection 2. Further, some animals at birth have the use of their natural powers, as the lamb at once flees from the wolf. Much more, therefore, would men in the state of innocence have had perfect use of reason at birth.
On the contrary, In all things produced by generation nature proceeds from the imperfect to the perfect. Therefore children would not have had the perfect use of reason from the very outset.
I answer that, As above stated (84, 7), the use of reason depends in a certain manner on the use of the sensitive powers; wherefore, while the senses are tired and the interior sensitive powers hampered, man has not the perfect use of reason, as we see in those who are asleep or delirious. Now the sensitive powers are situate in corporeal organs; and therefore, so long as the latter are hindered, the action of the former is of necessity hindered also; and likewise, consequently, the use of reason. Now children are hindered in the use of these powers on account of the humidity of the brain; wherefore they have perfect use neither of these powers nor of reason. Therefore, in the state of innocence, children would not have had the perfect use of reason, which they would have enjoyed later on in life. Yet they would have had a more perfect use than they have now, as to matters regarding that particular state, as explained above regarding the use of their limbs (99, 1).
Reply to Objection 1. The corruptible body is a load upon the soul, because it hinders the use of reason even in those matters which belong to man at all ages.
Reply to Objection 2. Even other animals have not at birth such a perfect use of their natural powers as they have later on. This is clear from the fact that birds teach their young to fly; and the like may be observed in other animals. Moreover a special impediment exists in man from the humidity of the brain, as we have said above (Question 99, Article 1).
Source: Summa Theologica (New Advent)