John Calvin Commentary Numbers 30:2

"When a man voweth a vow unto Jehovah, or sweareth an oath to bind his soul with a bond, he shall not break his word; he shall do according to all that proceedeth out of his mouth." — Numbers 30:2 (ASV)

If a man vow a vow. Wishing to modify the general law, so that no one would think there was any contradiction in this exception, he begins by repeating the law itself: that everyone should faithfully pay whatever he had vowed. This is as if to say that this principle remains valid, but that he only refers to those who are their own masters, and that women or girls who are under the authority of another were not free to make vows without the agreement and consent of their fathers' or husbands'.

This preface, however, must be understood, as I have already pointed out, as applying to lawful vows, by which neither religion is corrupted nor the holiness of God’s name profaned. And certainly, unless what we offer is acceptable to God, there can be no obligation on the conscience.

Moreover, since a distinction is made here between males and females, it can probably be conjectured that boys of ten years old, although still part of their family, are bound by their promises. Therefore, I will not stubbornly contend about this, because it is better to leave undecided whatever is doubtful and disputable, as it is commonly called, on either side.