There are 2 faults in Figure 1 below. It is easy here to work out that fault 1 is the youngest as it offsets fault 2.
Figure 1. Relative ages of 2 faults in the Permian Hopeman Sandstone, Inner Moray Firth, Scotland.
What about joints though, where there is no shear movement ? The 2 joints in Figure 2 belong to different sets as they have different orientations, but which set is the youngest ? Note that joint 2 bends towards joint 1. This suggests that joint 2 is the younger one, as it feels the presence of joint 1 and is almost attracted towards it. Similar, I assume to fault growth histories.
Figure 2. Looking down on a sandstone bedding plane. The compass sits between 2 joints. The oldest is labelled 1 and the youngest is labelled 2. Fractured Devonian sandstones, Caithness, Scotland.
Another example is given in Figure 3 from a steeply dipping sandstone bed on the flank of an anticline in Azerbaijan. Joint 2 abuts against the older joint (1). Again, we see joints curving (3) towards the older ones, suggesting that they (3) are the youngest set.
Figure 3. Relative ages of joints on a steeply dipping sandstone bed on the flank of an anticline in Azerbaijan. Oldest to youngest (1 - 3).