St. Hippolytus of Rome (215 AD) “Baptize first the children; and if they can speak for themselves, let them do so. Otherwise, let their parents or other relatives speak for them.” The Apostolic Tradition 21.
Origen (post 244 AD): “the Church received from the apostles the tradition of giving baptism also to infants.” Commentary on Romans 5, 9.
In 252 AD, the council of Carthage condemned the opinion that infants must wait until the eighth day after birth to be baptized, as was the case with circumcision. St. Cyprian of Carthage, Letter 64 (59), 2.
And also…
“And many, both men and women, who have been Christ’s disciples from childhood, remain pure and at the age of sixty or seventy years…” Justin Martyr, First Apology, 15:6 (A.D. 110-165).
“And when a child has been born to one of them, they give thanks to God [baptism]; and if moreover it happen to die in childhood, they give thanks to God the more, as for one who as passed through the world without sins.” Aristides, Apology, 15 (A.D. 140).
“Polycarp declared, 'Eighty and six years have I served Him, and He never did me injury: how then can I blaspheme my King and Saviour?” Polycarp, Martyrdom of Polycarp, 9 (A.D. 156).
“For He came to save all through means of Himself–all, I say, who through Him are born again to God–infants, and children, and boys, and youths, and old men.” Irenaeus, Against Heresies, 2,22:4 (A.D. 180).
“I, therefore, brethren, who have lived sixty-five years in the Lord.” Polycrates, Fragment in Eusebius’ Church History, V:24:7 (A.D. 190).