When do you need impedance matching?
In general, you don't need to worry about impedance matching at any frequency - AS LONG AS YOUR TRACE IS SHORT ENOUGH. The rule of thumb is that the time of flight along the trace must be less than 10% of the rise (or fall) time of the signal being propagated. This is slightly different than Matt Young's answer, and it's important if your signal is digital. A 50 MHz square wave with sharp edges will have a rise time much less than the period of the fundamental, and particularly in logic this can cause real problems if the reflected component is too large. 

So a good, clean 50 MHz (20 nsec) square wave might have rise and fall times in the range of 2-3 nsec, and (at a rule of thumb of 8 inches/nsec for PCBs), you'd want to start worrying for trace lengths of 2-3 inches or so. See here, for example. This is, in some respects, a very conservative number, since it assumes that the transmitter has zero output impedance and the receiver has infinite input impedance, and the transmission line (pc trace) has no losses. While none of these are exactly true, being conservative is very, very good if you want to avoid unpleasant surprises.