The Qur'an says in Arabic:
واللائي لم يحضن" فعدتهن ثلاثة أشهر
"and those who never had menses" (because they are underage)
Quran 65:4
Tafsir from al-Islam.com
That's in brackets, the Arabic doesn't say under age.
Wa Al-Lā'ī Lam Yaĥiđna
wa ( وَ ) = and
Al-Lā'ī ( وَاللَّائِي ) = for those who
Lam ( لَمْ ) = did not (negation in past tense)
Yaĥiđna ( يَحِضْنَ ) = menstruate.
Yaĥiđna comes from the verbal root H-Y-D ( حيض ) which means "to menstruate".
The addition of prefix "Ya" and suffix "na" to the root "HYD" shows that the word is used in used in third person, feminine gender, plural, imperfective aspect.
The imperfective aspect, by itself lacks any tense feature. The tensed negatives like lam ( لَمْ ) (negation in past tense), lan ( لن ) (negation in future tense), laa ( لَ ) (negation in present tense) combined with imperfectives decide the tense in this case.
Thus lam Yadrus = He did not study.
In the verse 65:4, Lam Yaĥiđna = 'those who did not menstruate'.
Further the imperfective verb in the context of lam ( لَمْ ) (past tensed negatives) is in the Jussive mood.
That is correct, and it doesn't show it means prepubescent; actually, it does the opposite what you are trying to prove here. I highlighted pertinent parts in red.
Arabic word, "Lam" is the important word here. It is strong negative, in past tense, which is to emphasis something didn't, but normally it is expected to happen. It doesn't even mean in the past it didn't happen at all, it can also mean it didn't happen in a certain expected period of time, which is present in your example too "e did not study." Here is another example with a reference:
All in all, meaning is not that specific or narrow as some people make it to be whether muslims or non-muslims, it just says for those who did not (or do not) had mensuration, they should wait for 3 months to rule out any doubts. It can, technically, include young girls who did not experience mensuration, for whatever reason, but are old enough to get pregnant based on the complete verse.
Here are some people, who would twist this and argue for the sake of argument, but I explained it for those who want to know what is actually written in verse.