Section 50. To be eligible for examination for a second class fireman's license, a person must furnish evidence as to his previous training, and, except students attending all day state aided vocational high schools in the steam engineering course or students performing the duties of a first or second class fireman at said school, be at least eighteen years of age; provided, however, that no such license issued shall be used in employment by the holder thereof unless he is at least eighteen years of age.

To be eligible for examination for a first class fireman's license, a person must furnish evidence as to his previous training and experience and must have been employed in a boiler or steam power plant as a steam engineer, fireman, control room operator, watertender auxiliary operator or engineer's assistant for not less than one year, or he must have held and used a second class fireman's license for not less than six months.

To be eligible for examination for a third class engineer's license, a person must be a citizen or furnish proof of having filed a declaration of his intention to become a citizen of the United States, must furnish evidence as to his previous training and experience and must have been employed in a boiler or steam power plant as a steam engineer, fireman, control room operator, watertender auxiliary operator or engineer's assistant for not less than one and one half years, or held and used an equivalent license in the United States merchant marine for one year or used a current third class steam license issued by another state for one year or must have held and used a first class fireman's license for not less than one year.

To be eligible for examination for a second class engineer's license, a person must be a citizen or furnish proof of having filed a declaration of his intention to become a citizen of the United States, must furnish evidence as to his previous training and experience and must have been employed as an engineer in charge of or operating a steam plant or plants having at least one engine or turbine of not less than fifty horse power for not less than two years or held and used an equivalent license in the United States merchant marine for two years, or have held and used an equivalent license from another state for two years or must have held and used a third class engineer's license either as an engineer, assistant engineer, control room operator or a fireman for not less than one year, or must be a person who has held and used a special license to operate a first class plant for not less than two years, except any person who is a United States citizen and served three years as an apprentice to the machinist or boiler making trade in stationary, marine or locomotive engine or boiler works and who has been employed one year in connection with the operation of a steam plant, or any person who has a bachelor of science degree in engineering from any duly recognized school of engineering, who has been employed for one year in connection with the operation of a steam plant, shall be eligible for examination for a second class engineer's license.

To be eligible for examination for a first class engineer's license, a person must be a citizen or furnish proof of having filed a declaration of his intention to become a citizen of the United States, must furnish evidence as to his previous training and experience and must have been employed for not less than three years as an engineer in charge of a steam plant or plants having at least one engine or turbine of over one hundred and fifty horse power, or must have held and used a second class engineer's license in a second class plant for not less than one and one half years, or in a first class plant as assistant engineer for one and one half years or held and used an equivalent license in the United States merchant marine for three years or have held and used an equivalent license from another state for three years.

Every application for any grade of license, except a second class fireman or special application granted under the provisions of chapter one hundred and forty-six, shall be indorsed by an engineer or fireman holding the same or higher grade of license. The indorsee shall state his name, address, grade of license currently in force and that he has personal knowledge of the applicant's experience and trustworthiness. The license of an indorsee making a wilful falsification shall be suspended or revoked. District engineering inspectors of the department of fire services shall not indorse applications, except when applying the oath when the applicant is being examined.