Hi thelonius,
From the photo it looks like the cairn has a substantial base, far wider than the exposed rocks on top. This also appears to be embedded in the ground, and substantially overgrown.
This suggests a cairn of some age, and not modern. Most summit cairns that have been erected since people started climbing mountains for fun don't usually have that look.
Obviously, that's just my opinion based on viewing a few pictures on t'internet. However, from what I have seen of prehistoric cairns in real life, and comparing them to non-prehistoric cairns, I would say that your cairn looks more like a prehistoric one. Having said that, I don't know whether cairns were built throughout history; i.e. could it be 12 century for arguments sake?
Cheers,
TE.