How old would you say the spirit world is?

Eternity can have no beginning. And eternity, as with immortality, is something that cannot be proved. 

This world, and all with it, are eternal.

Spirits have the feeling of absolute permanence. Everything here [in the world of spirits] in these lands cries out against there being an end to this glorious life, and spirits in these realms have the assurance of those stupendous souls in the very highest realms.