Hmm so it's not just people reincarnating but objects coming through into this world too. Well we already knew that but I wonder if it's connecting to why people who remember their "past lives" are denounced as witches.
When you give demons civilization and social lives instead of making them mindless evil beings, suddenly the goal of eliminating the demons turns a lot more sinister. They want to commit genocide.
As a rule of thumb, we shall look at the "Half-your-age-plus-seven" rule. Since Erden is now 22, the rule gives us a minimum of 18. Since she's 19, this was the perfect time skip.