peshat level, is that how do we know that naarah really means a girl only of that age?
Sure, in Rabbinic Hebrew, that is the meaning of naarah, as opposed to ketana and as opposed to bogeret, and Chazal even make derashot and halachic conclusions on that basis, based on pesukim. But must a pashtan agree to this assessment?
I will give a counterexample, from II Kings 5:
naarah who is little, ketanah. And throughout the rest of the perek she is called naarah. How is this possible?
Chazal deal with this, and Rashi explains:
a young girl Heb. naarah ketannah, a young girl from the town of Naaran.Yet if we disagree with this assessment, then the term naarah might even encompass a very young girl.
Now, we might reject in locally in parshas Chayei Sara for other reasons, but still -- when you are going out to reject a midrash, it is questionable to base yourself on a midrashic assessment of a term as used elsewhere.