[Muffet's a bit perplexed at the assumption that this world would have been around for such a short time, but perhaps other worlds don't work on the same timescales as her own?]

I don't know about the world itself, but the Storyteller mentioned that before we arrived they'd been sleeping for about four thousand years because there wasn't anything in particular for them to do. While I can't be certain of their age, the fact that they referred to that timespan as 'taking a nap' makes me suspect that four thousand years is a relatively brief period to them, so they're likely quite a few thousand years old at least. They seem rather tied to their land, as well, so I don't think they would have been around before this world formed...

The Jormun seemed to know of the Storyteller already, so there's at least one mortal civilization around that predates their long nap.