Tertullian’s On Baptism 18 recommends delaying baptism, ‘principally’ in the case of little children. That is an objection to an existing practice, not a report that Christians had never heard of it. Around 253, Cyprian and sixty-six bishops answer a question about whether a newborn should wait until the eighth day. Their unanimous answer is no: divine grace should not be withheld from infants (Letter 58).
Other witnesses, including Origen and the Apostolic Tradition traditionally associated with Hippolytus, add to the case, though their transmission and dating require care. The responsible conclusion is not that every apostolic congregation certainly baptized infants in precisely the later manner. It is that infant baptism is explicitly attested and debated centuries before the Middle Ages, without being introduced as a recent novelty.
Primary sources
Tertullian, On Baptism 18: https://www.newadvent.org/fathers/0321.htm
Cyprian, Letter 58: https://www.newadvent.org/fathers/050658.htm