Metal nanowires (NWs) enable versatile applications in printed electronics and optoelectronics by serving as thin and flexible transparent electrodes. The performance of metal NWs as thin electrodes is highly correlated to the connectivity of NW meshes. The percolation threshold of metal NW films corresponds to the minimum density of NWs to form the transparent, yet conductive metal NW networks. Here, we determine the percolation threshold of silver NW (AgNW) networks by using morphological analysis and terahertz (THz) reflection spectroscopy. From the divergent behavior of carrier scattering time and the increase of carrier backscattering factor, the critical NW density at which crossover from Drude to non-Drude behavior of THz conductivity occurs can be unambiguously determined for AgNW thin films. Furthermore, the natural oxidation of AgNWs which causes the gradual reduction of the connectivity of the AgNW network is also realized by the THz spectroscopy. The selective oxidation of NW-to-NW junctions weakens the ohmic contact, and for AgNWs near a critical density, it can even lead to metal-insulator transition. The presented results offer invaluable information to accelerate the deployment of metal nanowires for next-generation electronics and optoelectronics on flexible substrates.