Section 75. Any probation officer whose whole time is given to the duties of his office shall, at his request, be retired from active service and placed upon a pension roll by the appropriate county retirement board, established under section twenty, or in the case of Suffolk county, the Boston retirement board, upon recommendation of the court upon which it is his duty to attend with the approval, in the case of a probation officer in a district court, the municipal court of the city of Boston, or the Boston juvenile court, of the county commissioners of the county in which the court is situated; provided, that he is certified in writing by a physician designated by such court to be permanently disabled, mentally or physically, for further service by reason of injuries or illness sustained or incurred through no fault of his own in the actual performance of his duty as such officer. Any such probation officer who has faithfully performed his duties for not less than twenty consecutive years, and who is not less than sixty years of age, shall be retired at his request without the aforesaid certification. Every probation officer shall be retired upon attaining the age of seventy.

No probation officer whose employment begins after June thirtieth, nineteen hundred and thirty-seven, shall be subject to the provisions of this section.