Commentary for Kiddushin 100:1

In both of these cases the first act does not cause the woman, or women to be betrothed because there was not a perutah’s worth given to each woman. When the man later sends gifts which are worth more than a perutah, we might have thought that these gifts can now act as the betrothal money. After all, he clearly intended to betroth her and she clearly agreed. However, the mishnah rules that since these gifts were not sent with the intent of effecting betrothal, they do not act as such. We also do not assume that the man realized that his first act of betrothal was invalid and that he is now sending betrothal money.

A minor cannot betroth a woman. Betrothal requires “awareness”, and minors legally lack the required “awareness.” Therefore, if he tries to betroth a certain woman, she is not betrothed. Even if he later on sends presents to the woman whom he tried to betroth, the presents do not effect kiddushin, for they were not sent as such but rather on account of the first act of betrothal, which was invalid.