Steno's fourth principle, cross-cutting relationships, states that a fault cutting through bedrock is younger than the bedrock being cut.
Steno's fourth principle, cross-cutting relationships, states that when a geologic event cuts across another, the event doing the cutting is younger than the one being cut.
In the case of a fault cutting through bedrock, the fault is younger because the bedrock had to exist first to be faulted.
This principle applies to faults that cut through any rock type, whether it is sedimentary, igneous, or metamorphic.