To delve into Eirin's age from canon info:
Eirin is older than Tsukuyomi (source? SSiB or CiLR?).
Tsukuyomi is a son of Izanagi. The three noble children, Amaterasu, Tsukuyomi and Susanoo, were born when Izanagi returned from his terrible quest to meet the dead Izanami, and cleansed himself. In other words, they were born quite some time after Izanagi and Izanami had created the islands of Japan.
According to Izanagi Object ~ Neo-traditionalism of Japan, Izanagi and Izanami created Japan 25 million years ago. So Eirin's age might be on a time scale comparable to that.
(If we consider the possibility that Eirin is Omoikane, according to some version of family tree of the gods, Omoikane is of the same generation as Izanagi and Izanami. Anyways, Omoikane is older than the three noble children.)
Kaguya, though, can be much younger than that, most likely born after the founding of Lunar Capital.
About whether the Lunarians are human - it's not as important as you may think. Japanese mythology, like many animist mythologies, don't differentiate much between humans and gods, or humans and objects, for that matter.
>Sagus
You have a point about whether humanity can determine things that existed prior to humanity's evolution, and I also find it troubling. But did you know that there really is a quantum physics theory that proposed the universe was in a Schrodinger's cat state, until humanity was evolved enough to observe it, thus retroactively making the entire history of the universe snap into focus?
What if the pre-human history of Touhou universe is inspired by this theory?
