For people who grew up in Christian homes and churches from infancy, and who have always believed and practiced the Christian faith, there are often two ways to answer "When did you become a Christian?" One answer is that they've been Christians from the time they were born, because they have loved and followed Jesus for as long as they can remember. Another answer is that they became Christians when they were old enough to make a commitment to Jesus for themselves, a conscious decision to continue to follow in the path of Christianity.
It's reasonable for churches to mark both stages of life -- being born into a Christian environment, and making one's own decision.
Some churches use baptism as the rite through which one makes one's own commitment to Christ. These churches often have a dedication ceremony for infants.
Other churches use baptism as the rite through which an infant is accepted into the Christian community, to begin the Christian journey. These churches often have a rite of confirmation later in life, through which a person declares their decision to follow Christ.