I honestly don’t remember where I saw this fic, but it wasn’t long ago. I think it’s more of a narrative, where Sam and Dean are hunting through the years and they come to realize that 1: they aren’t aging like everyone else and 2: either the hunts are becoming easier to the monsters they hunt are becoming afraid of them. I’m pretty sure they also have powers, I remember a line where their guns no longer run out of bullets or something to that effect.