Age of spirits...

Hi,

I have a question... if it's in the wrong place, feel free to move it...

When hunting or even just minding your own and seeing, ghosts, how old can they possibly be?

For instance, here in New Mexico there are ghost or apparition sightings of Natives (for instance Philmont Scout Ranch's ghost riddled Mesa- PM for story) that must be almost 1000 years old, but for the most part ghosts are reported to be dead for less than 500 years.

It would make sense, for instance, for Teutoberg Wald, where 3 legions were completely destroyed in AD 9 by ambush, and many, many violent deaths occured over a 2 day period, to be a very haunted place. Yet I've never heard of a Roman ghost.

So I guess my question is, what is the basic timeline that experience would tell you is prime for ghost sightings?