Sec. 7. RETIREMENT PENSIONS. Whenever any member of said departments who shall have contributed a portion of his wages, as provided herein, shall have served twenty-five (25) years or more in either of said departments and shall have attained the age of fifty (50) years, he shall be entitled to be retired from said service upon application, and shall be entitled to be paid from said fund a monthly pension of one-half (1/2) of the salary received by him at the time of his retirement subject to change under the provisions of Section 10A of this Act.