Concerning the currency being called yen, I called it an underdeveloped world earlier because they use medieval armaments, but that's a 20th century home Aria lives in! Looks like I was far too hasty in the currency issue. For some inexplicable reason the folks just like to massively risk their lives when fighting the monsters, but otherwise they do have a high level of technology and thus maybe also a developed economy/financial system as well. It's still an intential joke by the author to call the currency yen, but otherwise with that level of development, it's perfectly viable to sport a fiat currency.
More importantly, though, I'm starting to think that world has a big problem with body snatchers. This is just the third chapter, but already a second person is suspicious of the MC. It's like the people are conditioned to look at children/youth and spot behavior that seems too mature. Why? Because they are afraid of some monsters/witches/sorcerers that take over the body of a child to live another life. Hard to think of another explanation. Unless folks actually know of reincarnators and aren't too thrilled about them or maybe the government pays a lot if such an individual is delivered to the officials.