After identifying the parameters of the piezoelectric transducer, the mechanical impedance of the front medium is deduced. More particularly, the wave propagation velocity and attenuation are deduced in the inspected plate. By fitting the electrical impedance measurement results, the aging of composite materials is quantified, showing the effectiveness of this means of nondestructive evaluation. A specific measurement tool and protocol are proposed. Several estimators are identified on the basis of Gaussian fits of the resonances observed on the electrical impedance measurements. Those estimators are identified and the obtained results show that both the frequencies and widths of the resonances peaks vary according to the health of the plate, or aging duration. This method is applied successfully on non-perfect sandwich plates, porous, and with non-ideally flat surface. The sensitivity and limits of the most relevant estimators are discussed for the two studied plate families.