puella ‧ From puellus ‧ a little boy ‧ , contracted from puerulus ‧ a little boy, a little slave” ‧ the diminutive of ‧ puer ‧ a child, boy, lad, servant, page, slave ‧ From PIE Proto-Indo-European *ph₂weros, from peh₂w- Cognate with Oscan (puklum), Ancient Greek παῖς ‧ [ paîs ‧ child ]
femina ‧ From Proto-Italic *fēmanā, from PIE Proto-Indo-European [ *dʰeh₁-m̥h₁n-éh₂ (“(the one) nursing] ‧ feminine mediopassive participle of dʰeh₁(y)- [ to suckle, nurse ]. Related to fīlius, fellō, fētus.
So many cognates from the PIE root [ dʰeh₁(y)- ]: Doe, Dairy, Dam, Dame, Děvica, Дама, Дева, Дети, Дои́ть, Fawn, Fecund, Female, Fetus, Felix, Felicity, Felon, ..., Teat, Tia
PIE root dʰeh₁(y)- derived terms:
English: dey ‧ From Middle English deye, deie, daie, from Old English dǣġe (“maker of bread; baker; dairy-maid”), from Proto-Germanic daigijǭ (“kneader of bread, maid”), from Proto-Indo-European dʰeyǵʰ- (“to knead, form, build”). Cognate with Swedish deja, Icelandic deigja (“dairy-maid”); compare dairy, dough, lady. ‧ (Britain dialectal, Scotland) A servant who has charge of the dairy; a dairymaid.
Slavic: dětь to suckle [ dětę děti ‧ child children ] ‧ [ děva (“maiden, girl”) ] ‧ [ dojiti (“to give milk ]
Hellenic: Ancient Greek: θηλή (thēlḗ, “teat”), θεῖος (theîos, “uncle”), τίτθη (títthē, “nurse”), τήθη (tḗthē, “grandmother”), Τηθύς (Tēthús)
Indo-Iranian: Sanskrit: धेनु (dhenu, “cow”)
"femina" could be used for anything feminine, so also a child or even animals. The more frequent and specific word for woman is "mulier". To the contributors to this course, I'd highly recommend replacing femina by mulier, not just in this sentence but in general.
Am I right in thinking Latin has no concept of definiteness? Would "the girl is not a woman" be the same in Latin?
Latin has no articles, so it could be the same. If you want some more definiteness, I think you could use words like "ille/illa" (meaning "that") or something like that (I'm not sure), but it would not be used all the time. My native language, Czech, works more or less the same way.