Section 78. Any laborer employed by any fire, water or sewerage district, or a joint water board of two or more municipalities, hereinafter called a water board, which accepts this section prior to January first, nineteen hundred and forty-six, or has accepted corresponding provisions of earlier laws, who has reached the age of sixty and has been in the employ of such district or water board for not less than twenty-five years and has become physically or mentally incapacitated for labor, and any laborer in the employ of any such district or water board who has been in such employ for a period of not less than fifteen years and has become physically or mentally incapacitated for labor by reason of any injury received in the performance of his duties for such district or water board, may, at his request, with the approval of the prudential committee, water commissioners, water board or sewerage commissioners be retired from service; and if so retired he shall receive from the district or water board, for the remainder of his life, an annual pension equal to one half of the average annual rate of regular compensation for full time service paid to him as a laborer at his retirement. Any laborer in the employ of such a district or water board who has reached the age of sixty-five and has been in such employ for not less than twenty-five years shall be retired from service, and shall receive from the district or water board an annual pension computed in the manner hereinbefore set forth. In the event that a district or water board has succeeded or shall succeed to the water or sewage disposal works of any city or town, the time of service of any laborer on any such works, who has become or shall become an employee of such district or water board, shall be computed as having been rendered to the district or water board. This section shall take effect in any fire, water or sewerage district if accepted as aforesaid by the district at a meeting, and shall take effect with respect to a water board if accepted as aforesaid by such board.