The license fee for a taxi operator’s license shall be five dollars per year, and shall be tendered with the application; provided, that all licenses shall expire on December 31st of the year in which they are written. An operator’s license or a receipt from the city clerk showing that the license has been granted must be carried in the vehicle or on the person of the operator. Taxi operators’ licenses shall not be transferable. All taxi operators must be at least twenty-one years of age.