הבחורה ישנה is also correct, I think.
Yes, a בחורה is older. "Young lady", you could say.
Is יושנת (yoshenet) also acceptable?
This is what I want to know. My understanding of pa'al present tense is that you add a 'ו' after the first letter of the root word.
Is ישנה at all related to the adjective, ישן?