פירוש על קידושין 86:7

The Talmud quotes a mishnah from Gittin. According to the first opinion, a betrothed girl who is between 12 and 12.5 years of age, a na’arah, can either receive her own get or her father can. R. Yehudah holds that only her father can receive the get. And if she is so young (or mentally incapacitated) that she cannot hold on to her get, then she cannot be divorced.