A: Sentient beings - at least humans and minor human variants - have lived in the Aether as long as it has existed, because they were the ones who created it. The base population of the Aether migrated to it from Earth.
As for actual length of time, that is more up for debate. Thanks to disasters disrupting the recording of history and multiple breaks in time between the Aether and the Earth, the Aether may or may not be thousands or millions of years older than the Earth, despite being chronologically younger.
Due to substantial loss of records [i.e. I'm not sure yet], it is not publicly known exactly when the Aether was created, but a guess would be around 2500-2000 BCE. Will go into more detail on this later.