Every person now or hereafter employed by the said cities, or paid out of the city treasury thereof, as herein above provided, if any, of the age of sixty years and upwards, who shall have been so employed for a period of twenty years or more, shall, upon application to the board of pensions herein created, be retired from service, and shall during the remainder of his or her life receive a pension or compensation fixed by this act, subject to such qualifications as are herein contained: Provided, That if any person or persons had been employed by the said cities for a period covering eighteen years or more prior to the passage of this act, and shall thereafter be reemployed, it shall be necessary that the period of their reemployment shall extend over a period of two years or more before such person or persons shall be entitled to receive the pension fixed by this act.