Christening is baptism with a naming attached, and the question of whether infants should receive it is the longest-running unresolved argument in Christian practice. Both sides have a coherent case and neither has ever persuaded the other.
The churches that baptise infants argue that grace is prior to response. Whole households are baptised in Acts without any note of who in them could speak. The covenant sign in the Hebrew Bible was applied to eight-day-old boys, and the church that reads itself as continuous with Israel sees no reason to require of infants what was never required before. Augustine sharpened this into an argument from original sin that made infant baptism close to urgent, and the practice of emergency baptism by a midwife, valid without any clergy, follows directly from taking that seriously. Godparents exist to make the profession the child cannot.
The churches that do not baptise infants argue that the New Testament shows baptism following a confession of faith, that nothing can be professed on another person's behalf, and that a rite administered to someone incapable of meaning it is not the rite at all. The sixteenth-century Anabaptists took this position and baptised adults who had already been baptised as infants, which their opponents treated as a capital offence; the name that stuck to them means rebaptisers and was an accusation. Baptist, Mennonite and most Pentecostal churches hold the position today.
The consequences are not confined to theology. Because the second group does not regard an infant baptism as a baptism at all, and the first regards rebaptism as a serious wrong, the two cannot fully recognise each other's members without one of them conceding. Most ecumenical agreements of the last century have managed the disagreement rather than resolved it.
A quieter difference sits alongside it. The Orthodox churches baptise, chrismate and give communion to an infant in a single continuous rite. The Latin church spreads the same three actions across a decade or more. That is a disagreement about whether Christian initiation is one event or three, and it has nothing to do with the age question at all.