Now I'm coming late to the party here, and I've not read the zillions of other posts, so apologies if I'm contradicting or repeating info elsewhere in this thread.

Yes it is possible to age new carvings to some extent. It takes bloody ages, patiently wearing down all the peckmarks. Then leave it outside long enough for a coating of yoghurt to turn into moss then lichen, and it'll look not new.

And it can be possible to differentiate different periods of carving on the genuine article. Compare the newer motifs at Hunterhegh 1, where the newer carvings have a different kind of peck mark, and the old ones are very much more weathered, with Hunterheugh 2 a few yards away, and you can see old carvings that have been partially exposed to the elements, so the upper part of the panel looks worn, but lower parts of the same motif are fresh and show all the peck marks clearly.