The protein digestibility-corrected amino acid score (PDCAAS) has been recommended to be the most suitable method for routine evaluation of protein quality of foods by FAO/WHO. The PDCAAS method includes the use of young rats for predicting protein digestibility of foods for all ages including the elderly. To assess the usefulness of protein digestibility in old rats in the calculation of PDCAAS for the elderly, the influence of age on the digestibility of protein in 5-wk-old and 20-mo-old rats by the balance method was studied. Fifteen protein products were tested. Each protein product was fed as the sole source of 10% dietary protein. A protein-free diet was also included to obtain an estimate of metabolic fecal protein. Protein digestibility values (corrected for metabolic fecal protein loss) in old rats were significantly (P < 0.05) lower than in young rats for most products; however, these differences were small (up to 3%) for properly processed animal products (casein, whey protein concentrate, whey protein hydrolysate, lactalbumin and skim milk powder). Similarly, the differences attributed to age were not large (up to 5%) for properly processed vegetable protein products (soy protein isolate and autoclaved soybean meal, black beans and fava beans). However, digestibility values in old rats were considerably lower (7-17%) than in young rats when fed products containing antinutritional factors, that is, mustard flour containing glucosinolates; alkaline/heat-treated soy protein isolate and lactalbumin-containing lysinoalanine; raw soybean meal and black beans containing trypsin inhibitors; and heated skim milk powder containing Maillard compounds. Therefore, the inclusion of protein digestibility data obtained using young rats in the calculations of PDCAAS may overestimate protein digestibility and quality of these products for the elderly. For products specifically intended for the elderly, protein digestibility should be determined using old rats.