Yevamot 35A

וגזרו רבנן קטנה משום גדולה.

And, although there is no possibility for her to become pregnant, the rabbis issued a rabbinic decree requiring the 3 month waiting period for a female minor due to this requirement for a female adult who engaged in promiscuous sexual acts.

ומי גזרינן קטנה משום גדולה? והתנן: אם היו קטנות שאינן ראויות לילד ⁠— מחזירין אותן מיד. אמר רב גידל אמר רב: הוראת שעה היתה. מכלל דהואי? אלא: כהוראת שעה היתה. וחילוף לא שכיח.

The Talmud asks: And do we issue a decree with regard to a female minor due to the ruling for a female adult? But didn’t we learn in the Mishnah: If they were female minors who could not bear children, we return them immediately to their husbands? This indicates that there is no concern for pregnancy, and the rabbis did not issue a decree in this case. Rav Giddel said that Rav said: This was a provisional edict issued in exigent circumstances, and therefore one cannot extrapolate from the case in the Mishnah to other situations. The Talmud wonders: Can one assume by inference that there was such an occurrence? It would seem from the Mishnah that this was merely a possibility and not an actual occurrence, for if it actually happened it would have been appropriate for the Mishnah to relate the actual case. Rather, the ruling in the Mishnah is like a provisional edict in that switching of wives, such as described in the Mishnah, is uncommon, and in cases that are not common, the rabbis do not issue a decree. Therefore, in the case of the Mishnah, the female minors were not required to wait.

לישנא אחרינא, אמרי לה אמר שמואל: כולן צריכות להמתין שלשה חדשים, חוץ מגיורת ומשוחררת גדולה. [אבל] קטנה בת ישראל אינה צריכה להמתין שלשה חדשים. במאי? אי במיאון ⁠— האמרה שמואל חדא זימנא. אי בגט ⁠— הא קאמר שמואל דבעיא! דאמר שמואל: מיאנה בו ⁠— אינה צריכה להמתין שלשה חדשים! נתן לה גט ⁠— צריכה להמתין שלשה חדשים! אלא בזנות, וזנות בקטנה לא שכיח.

Some say another version of what was taught: Shmuel said: All women must wait 3 months, except for a female convert who is an adult and a freed female slave who is an adult; these women need not wait. However, an Israelite female minor need not wait 3 months in any case. The Talmud clarifies: With regard to what situation is this statement referring? If it is referring to a female minor released from her marriage by refusal, it would be superfluous, as didn’t Shmuel already state this halakha one time? If it is referring to a woman released by a bill of divorce, but didn’t Shmuel say that in that case she is required to wait, as Shmuel said: If she refused him she need not wait 3 months, but if he gave her a bill of divorce, she must wait 3 months? Rather, this is referring to cases of promiscuous sex, and an occurrence of promiscuous sex with female minors is uncommon, and the rabbis did not issue rabbinic decrees in uncommon instances.