It was in the year III 1999 that Thráin I led his people to Erebor to found the Kingdom under the Mountain, so that would be the earliest point in history when the Ravenhill guard-post might have been built. Of course, it might easily have been built after this time (and perhaps not until after the recolonisation of Erebor by Thrór in III 2590, or even later).

The only oblique references to its age that we have is in The Hobbit, where it's called an 'old watch-post' (chapter 11, On the Doorstep), and it's remarked that Thorin remembered the post being in place since he was a young Dwarf. This would make it at least two hundred years old when Bilbo and his companions reached it during the Quest of Erebor, but possibly far older than that.