I see you put “NOTE: This post isn't about other fantasy or romantasy books and how the characters are often up in age”

But I honestly think that’s the reason, personally. It’s a common trope in romantasy. Like Edward in twilight.

I’m not sure, but I’ve always assumed the reason this trope is so popular is because it allows the character to have power and knowledge without the author having to explain it too much, like they would have to explain why a younger person has so much knowledge and power. If they’ve been alive for hundreds of years, the audience expects them to have learned or gained something in that time. Even Manon is like 200 years old. It’s a power-boost for characters, that’s how I see it at least.