The bonding interface between the self-compacting concrete (SCC) filling layer and the steam-cured concrete precast plate plays an important role in the durability of the China Rail Track System III (CRTS III) slab ballastless track structure. It is of great practical value to accurately detect the quality of the bonding interface. This paper investigates the influence of age on the detection of bonding interface defects via the impact-echo method. For different ages of the SCC filling layer, the stress nephograms produced under the same impact load are investigated by the finite element method. Via full-size field experiments, the results of the bonding interface quality detected by the impact-echo method are compared with the actual quality state determined by the field test. It is found that there is a good correlation between the age of the SCC filling layer and the bonding rate of the bonding interface. The research also shows that the existence of a bonding interface cavity changes the vibration mode. This work provides important technical support for the detection of the quality state of the bonding interface.