Why do we baptize infants?
It would seem that baptizing infants is kind of weird or even stupid considering that infants can’t choose to be baptized. Why not wait until someone is old enough to choose the sacrament to confer it on them? The best answer to this question is to recognize Baptism as a spiritual birth. Just like physical birth, the baptized infant relies on another for his baptism. The baptized infant receives his salvation by an act of Faith on the part of the Church and the willing consent from the parents and representation of the godparents. (Aquinas, ST III. Q68, A9).