Every person now or hereafter employed by the said cities, as herein provided, if any, if 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 the pension or compensation fixed by this act, subject to such qualifications as are herein contained: Provided, That if such person had been employed by the said city for a period covering eighteen years or more prior to the passage of this act and shall thereafter be re-employed, it shall be necessary that the period of his or her re-employment shall extend over a period of two years before such person shall be entitled to receive the pension fixed by this act: And provided further, That any person employed by said city for a period for more than twenty years shall be allowed a deduction of one year from the age herein fixed for retirement for each five year period of service in excess of twenty years.