I just think of each zone being frozen in time for their expansion.
Outside of specific quests, Ul'dah takes place during ARR, Ishgard takes place during HW, etc.
There's no need to worry about NPCs not aging because it's understood to be a slice of time.
The twins are really the only standout that feel weird to never age when a significant amount of time has clearly passed, but even there, they can get away with it until it becomes unbelievable that less than 5 years have passed since Elezen don't hit their growth spurt until 20-ish.