Christ Himself teas not baptized in His infancy, but as an adult.

The Catholic Answers

Christ was baptized as an adult because only then did He institute this essential rite of the New Law. You could not expect Him to receive it before instituting it. Yet remember that He had received the rite of Circumcision which is figurative of the Baptism to be received by Christian children, and that rite He received in infancy.

Radio Replies Volume 1 by Rev. Dr. Leslie Rumble MSC and Rev. Charles Mortimer Carty