Yoru's ass wasn't what was aged though, it was her face. The reason why the effects got reverted, despite them being physical changes, is because devil powers have in TVTrope Speak-
No Ontological Inertia, which is when effects are negated when their causes are negated, even if the negation happens after the cause caused the effect. Yoru went back into Aging's world where devil powers don't work, she was aged by a devil's power, so now she's fine.
This is also why I still hold that the Zombie Devil never died, as it's explicitly laid out that devil powers fade after the Devil dies. Just because Zombification is a mutation doesn't mean it's exempt from that.
Upscaling above the Gun Devil to
Town level+ was really funny to read though ngl.