Jamieson, Fausset and Brown on Numbers 30:3

3. If a woman also vow a vow unto the Lord, and bind herself by a bond, being in her father's house in her youth--Girls only are specified; but minors of the other sex, who resided under the parental roof, were included, according to Jewish writers, who also consider the name "father" as comprehending all guardians of youth. We are also told that the age at which young people were deemed capable of vowing was thirteen for boys and twelve for girls. The judgment of a father or guardian on the vow of any under his charge might be given either by an expressed approval or by silence, which was to be construed as approval. But in the case of a husband who, after silence from day to day, should ultimately disapprove or hinder his wife's vow, the sin of non-performance was to be imputed to him and not to her [Nu 30:15].