53 Pa. Stat. § 769
Each ordinance or resolution establishing a police pension fund shall prescribe a minimum period of total service in the aggregate of twenty-five years in the same borough, town, township or regional police department and shall fix the age of the members of the force at fifty-five years, or, if an actuarial study of the cost shows that such reduction in age is feasible, may fix the age of the members of the force at fifty years, after which they may retire from active duty, and such members as are retired shall be subject to service, from time to time, as a police reserve, in cases of riot, tumult or preservation of the public peace until unfitted for such service, when they may be finally discharged by reason of age or disability.
53 P.S. § 769