Retirement of a member on a service retirement allowance shall be made by the board of trustees as follows:

(1) Any member in service may retire upon his written application to the board of trustees setting forth at what time not less than thirty days nor more than ninety days subsequent to the execution and filing therefor, he desires to be retired; provided, that the said member at the time so specified for his retirement shall have attained the age of sixty, and notwithstanding that, during such period of notification, he may have separated from service;

(2) Any member in service who has attained the age of seventy shall be retired forthwith provided that upon request of the board of police commissioners the board of trustees may permit such member to remain in service for periods of not to exceed one year from the date of the last request from the board of police commissioners.