The only way to approximately date stones is to dig under them and date any
artifact found there. If any are found and dated, all that does is give a
date for the artifact - the stone could have arrived later. The 'date' would
be the earliest possible (from the artifact) and 'not before', so you would
say that it was placed in position sometime after the date of the artifact.
Similar stones can then be reasonably assumed to be of the same age.
If anyone recalls the 'Time Team' episode where they investigated a site in
Wales that they found out to have been 'salted' they'll know what I mean
(the La Tene sword on TOP of a piece of barbed wire springs to mind LOL)