2,815. The Liability of Minors Who Vow

Hilchos Nedarim 11:4

Once children have reached the age of majority, their vows are effective even if they have not yet developed the physical signs of maturity, meaning that they aren’t yet considered adults in all matters. The idea that when someone near the age of majority makes a vow, his consecrations and vows are effective is of Biblical origin. However, while their vows are effective, if they violate their vows or if they make oaths and substitute for them, they are not subject to the penalty of lashes until they reach the age of majority and develop the physical signs of maturity.

Hilchos Nedarim 11:5

If he sanctified something and an adult benefited from the thing that he sanctified, the adult is liable to the penalty of lashes because vows made by such a minor are effective by Biblical law, as has been discussed.