Acoustic nonlinearity measurements via contact and noncontact generations of surface acoustic waves (SAWs) were performed in order to characterize the thermal aging of aluminum alloy. The experiments were conducted on aluminum alloy samples (Al6061-T6) that were heat-treated at 220°C for different times (0 min, 20 min, 40 min, 1 h, 2 h, 10 h, 100 h, 1,000 h) and thus had the different levels of thermal aging. The acoustic nonlinearity of the specimens in two types of SAWs was observed according to the thermal aging. The fractional changes in the acoustic nonlinearity exhibited similar trends in both contact and noncontact SAWs, showing that the acoustic nonlinearity measurement via SAWs is independent of the SAW-excitation method. Furthermore, the fractional changes agreed well with the variation in the yield strength, which was a minimum when the acoustic nonlinearity reached its first peak. Then, the acoustic nonlinearity drastically dropped while the yield strength increased to its highest value. Thus, the variation in the acoustic nonlinearity can be perceived as an indicator of the aging level. These results demonstrate the potential feasibility of acoustic nonlinearity measurements via SAWs for the nondestructive evaluation of material degradations.