The present invention relates to a beef-specific age determination marker containing the p21 protein, to an antibody specifically bound to bovine p21 protein, to a beef-specific age determination kit containing the antibody which is specifically bound to the bovine p21 protein, and to a method which involves detecting the bovine p21 protein through an antigen-antibody binding reaction using the antibody which is specifically bound to the bovine p21 protein serving as a beef-specific age determination marker in the muscle tissue of beef, so as to determine the age of the beef. According to the present invention, the p21 protein is significantly greatly expressed in the muscle tissue of beef, the age of which is lower than 30 months, and is hardly expressed in the muscle tissue of beef, the age of which is greater than 30 months, and thus can be valuably used as a beef-specific age determination marker.