Whenever this chapter becomes operative in any county, the board shall order that any person who has been duly appointed, selected, or elected and qualified and has served as a forester, firewarden, or fireman for 20 years or more in the aggregate for which contributions provided for in this chapter have been made, and who has reached age 60, or who thereafter completes such service and attains age 60, shall be retired as of the first day of the succeeding calendar month.