Rashi on Leviticus 27:5/רש"י על ויקרא כ״ז:ה׳

1ואם מבן חמש שנים AND IF IT BE FROM FIVE YEARS OLD — Not that the person making the vow is a minor (from five to twenty years old), for there is no validity in the words of a minor, but it is speaking of an adult who said, “The payment of the valuation of this child” — who is five years old — “shall be incumbent upon me”.