Albert Barnes Commentary Numbers 30:3

"Also when a woman voweth a vow unto Jehovah, and bindeth herself by a bond, being in her father`s house, in her youth," — Numbers 30:3 (ASV)

Being in her father’s house in her youth: It was not ordinarily until her betrothal or marriage, that the female passed (some suppose by purchase) from the power of her father to that of her husband.