Any employee of the city who has reached the retirement age or who is unfit, physically or mentally, to continue in the service of the city, upon the request of the executive officer of the city, shall retire from active service, and, if the employee shall have been employed by the city for periods totaling one hundred twenty months or longer, the board of trustees shall make provision for the payment of a pension, such pension to be in a sum equal to the proportion of a full pension as herein provided which the total number of months employed by the city bears to two hundred forty months.