How Old Is Rimuru Tempest in Hunter X Hunter

It's a common query, but let's clear up a significant point right away: Rimuru Tempest, the beloved slime protagonist, is not from Hunter x Hunter. He hails from the incredibly popular anime and light novel series, That Time I Got Reincarnated as a Slime (also known as Tensura). This crucial distinction is the first step to truly understanding his unique existence. So, if he's not a Nen user, just how old is this powerful, shape-shifting slime?...