Frequency-dependent terahertz (THz) penetration depth is experimentally assessed using a multilayered water-skin model. The skin adsorbed with one water overlayer is considered as the multilayered structure to reflect THz waves and the reflected THz field variation is experimentally observed during a water desorption process. The skin penetration depths can thus be derived using the multilayered water-skin model while considering the measured reflectivity, water dielectric constants, and effective thicknesses of the adsorbed water overlayer. The largest penetration depth is independent of the illumination power of THz wave and in 0.4-0.6 THz at approximately 0.3 mm, covering the possible depths of stratum corneum (SC) and partial epidermis. The distinctly high penetration depth of 0.4-0.6 THz waves is finally confirmed via drying and damaging the SC.