What is the ruling on a grandmother's vow to marry off her grandson when he reaches the age of thirteen, knowing that he has exceeded this age and is not qualified for marriage?

Marrying off young people is desirable in Islamic law. The vow made by that woman to marry off her grandson when he reaches a certain age is a vow concerning a permissible act, regarding which scholars have differed on its validity. The majority of them hold that it is not valid, and this is the preferred opinion. Consequently, nothing is binding upon the woman, though if she offers an expiation for an oath, it is more cautious.