The literal meaning of infant, from the Latin word info’s, is “one unable to speak.”
Though that literal sense actually applies only to the first year or two of babyhood, both under Roman law and those of our own country a person is referred to as an infant until he has reached such age as he may legally enter into certain forms of contract.
That is, extending the literal sense, a person remains an infant until fully able to speak for himself in all matters.
In Italy, some centuries ago, much as we today apply the word “maid” to any female servant, there began the practice of calling a personal attendant of a knight, or a foot soldier, or other similar retainer an infante. Possibly, though it is not known, the practice came about through the youthfulness of the early retainers.
A collection of retainers or foot soldiers became infanteria, a term that, passing through French, produced our infantry.