Dov, if there were an 80-year-old man, a 40-year-old man and a teenage girl, you would say “the young man sees a pretty girl”, and you would mean the 40-year-old man as the young man but he is no longer נער.
And if your sentence was something written in the 50s or 60s, even women in their 40s or so were referred to as girls, so your reader might be confused because a 45-year-old woman is no longer נערה.
You are correct, but I was really only referring to the English translation "teenage boy" as sounding clunky. Maybe I'd say "the teenager sees a beautiful girl" and leave it to the reader to fill in that the girl is also approximately the same age. It's just a question of writing style. The problem is that the English here is an exact translation, and is technically correct, but sounds odd to my ear. You know how you sometimes read technical manuals for products manufactured in some foreign country and you somehow know that the manual writer is translating literally from his/her language, because the sentences sound awkward? That's what I'm driving at here. There's simply a better way to have translated this sentence and still get across the idea that this is a teenager seeing a pretty girl, without sounding like a non-native speaker.