How old is Yvonne?? Like mentally, all together, both lives.
I'm not sure we ever get any clue how old Yvonne was before, other than old enough to have a job.
I would think that she is at least 20?
If you just take her minimum age at isekai, say 16, and then add life in this world, that'd be 32+, or 26+ if you just count from when her memories returned. But I don't know if living childhood twice -- and having the biological brain of a child, or by now, teenager -- should be counted the same as being an adult of that timespan.
If she got isekaied at 16, then she's actually never been biologically older than she is now, she just has somewhat more overall experiences -- but likely not of sex or relationships.
Come to think of it, I think the most pointed criticism of this work might be how it treats the isekai element. Yvonne has pretty good memories of the game (though as a beta tester, never knew that much about it), but hardly ever thinks about anything else. From the novel (no big spoilers) there is a bit: she's got cooking and cleaning skills, vague mentions of jobs to buy stuff, she seems more comfortable taking care of herself than having servants, now we have this "oh no age gap". But I recall no mention of family, friends, college, living by herself, etc, nor her being introspective about having limited memories, say.
Contrast to other works:
IFTV: Rei has excellent memories of her past self, which seems to be the dominant personality after cascading at age 15 or so. She remembers the game world, but also being an overworked OL, her time in school, her friends and past relationship -- in fact this all becomes plot relevant. (Even the OL skills are mildly relevant!)
MagiRevo is at the other end: around age 6 Anis got some memories of Earth, but it feels like the memories were hazy, and that the native Anis is still the dominant personality. She doesn't think much about her past life because it's not clear she remembers all that much from her past life, other than the possibility of flying machines and home appliances.
Yvonne remembers the game well, like Rei, but not much else comes up. To be fair, she has been dual-memory in this world for 10 years, so it's possible the Earth memories have simply been pushed down and atrophied, except for the game system stuff which is unavoidably relevant to her current life.