Surah talaq 65:4 kindly review tafseer already qouted and the below

[islamquest]

According to hadith the corrective understanding of the trafseer "not menstruating" does not mean under age girl
Rather it means pure check the below hadith

Sunan an-Nasa'i 3398
It was narrated from Ibn 'Umar that he divorced his wife when she was menstruating, but the Messenger of Allah told him to take her back, and divorce her when she was pure (not menstruating).

Sunan an-Nasa'i 3395 It was narrated that 'Abdullah said: "The Sunnah divorce is to divorce her when she is pure (not menstruating) without having had intercourse with her." It was narrated that 'Abdullah said: "Divorce according to the Sunnah means divorcing her with one divorce in each cycle when she is pure, then when she becomes pure the third time, then he pronounces divorce again, and after that she must wait one more menstrual cycle." Grade : Hasan (Darussalam)

Besides the under age marriage are not permissible in islam as below (marriage only upon age if puberty)

Jami` at-Tirmidhi 1109 Abu Hurairah narrated that: The Messenger of Allah said: "An orphan is to be consulted about herself, then if she is silent that is her permission, and if she refuses, then do not authorize it (the marriage) for her" (meaning: when she attains the age of puberty and refuses it.) Allah subhanahu wa ta'ala knows best may he forgive (ameen)