Oh Grow Up!
So, let's turn at last to a consideration of age in the Middle Ages. We'll move through the life cycle, from infancy to old age.
Infancy
When did infancy begin and when did it end? Even in the Middle Ages, the answers were not as clear-cut as you might think.
As for the beginning of life, that was fairly unambiguous: it was from the moment the child took its first breath. Until that time, the child was only potential life. Medieval people understood that a child grew in the womb, and Caesarean section was a known technique for giving birth, but even so, the life was not truly begun until that first breath.
Even then, the child wasn't quite a full person yet, for he had no name. If the child were sickly and seemed unlikely to survive, a doctor or any lay person could perform baptism, so that it would die Christian, but in general the child wasn't baptized for a week or so. This was partly so both mother and child might be strong enough to get to the church, and partly to ensure the godparents would be present. Once the child was baptized, only then did it have a name and a real existence. Children who were exposed or abandoned were not baptized and not named; in a sense, they did not yet exist.
Baptism was the key ceremony. It was attended not only by parents and godparents but by others as well; it was a ceremony of recognizing a new member of the community. The child was named at baptism, which is why your first name is sometimes called your "Christian" name. The wealthy of course wished their children to be baptized in style, and cities built magnificent baptisteries that are still marveled at by tourists.
So, infancy can be said to have begun at baptism. How long did it last? A year or two. One medieval catch phrase characterized infancy as "no language, no mobility, no teeth." Once these were acquired, the child was no longer an infant.