Gateposts near Carn Gyfrwy

Thanks to Phil Morgan for this excellent pic of a pair of spotted dolerite (?) gateposts on the southern flank of the Preseli ridge, in rough country near Carn Gyfrwy.

How old are they?  Could they be prehistoric standing stones that have simply been incorporated into a hedge line, or are they relatively modern?  And if they are modern, why didn't the people responsible use timber instead, which must have been in abundant supply?  Again, the only thing I can think of is that the stones were handily lying around, and that they were valued because they were long enough and heavy enough to give the stability needed for hanging a gate........