Further insights from the novel about Yvonne's age:
Not much, really. She's experienced the 8 AM subway rush, but high school or even middle school could have provided that, in Japan. She's never dated or been intimate with anyone before Elsa, so either she died young or had a very single life and isn't more experienced than Elsa.
The translation does say that she regained her past-life memories at 13 in this world; there was also a line suggesting that she doesn't have the earlier memories of this world, but that feels wrong and might have been an error on someone's part. But I dunno, maybe she woke up at 13 and had to fake her way via her knowledge from the game.
Contradictory lines: "she knew nothing of the fifteen years between her birth and the beginning of the main story nor did she regain her memories until late in." but "She had spent more than ten years constantly surrounded by servants,"
So it's possible she got trucked at 16-20, then added only 3 years after that in this world. Yvonne and Elsa are explicitly 16. Or almost 16.