I found this interesting in Gregory Nazianzen’s (329-389) Oration 40, “On Holy Baptism” [you can listen to this work in its entirety over at Maria Lectrix]. Note that he is not yet advocating full-blown infant baptism, but only when “danger presses.” He thinks that you can otherwise baptize as early as “three years,” once they understand something of the Sacrament.
XXVIII. Be it so, some will say, in the case of those who ask for Baptism; what have you to say about those who are still children, and conscious neither of the loss nor of the grace? Are we to baptize them too? Certainly, if any danger presses. For it is better that they should be unconsciously sanctified than that they should depart unsealed and uninitiated.
A proof of this is found in the Circumcision on the eighth day, which was a sort of typical seal, and was conferred on children before they had the use of reason. And so is the anointing of the doorposts, Exodus 12:22 which preserved the firstborn, though applied to things which had no consciousness. But in respect of others I give my advice to wait till the end of the third year, or a little more or less, when they may be able to listen and to answer something about the Sacrament; that, even though they do not perfectly understand it, yet at any rate they may know the outlines; and then to sanctify them in soul and body with the great sacrament of our consecration. For this is how the matter stands; at that time they begin to be responsible for their lives, when reason is matured, and they learn the mystery of life (for of sins of ignorance owing to their tender years they have no account to give), and it is far more profitable on all accounts to be fortified by the Font, because of the sudden assaults of danger that befall us, stronger than our helpers.