Any member of the fire department of any such city having served twenty years or more in such fire department, of which the last two years shall have been continuous, may have application to be relieved from such fire department and retired; and if his application is granted, or if he shall be discharged from such department, the said board of trustees shall order and direct that said person shall be paid out of the pension fund monthly such sum of money as may be determined by the rules and regulations provided for the management of said funds; and if he be a member of the volunteer fire department and not under pay, such amount monthly as may be fixed by the board of trustees. After the decease of such member, his widow, provided their marriage shall have occurred prior to such retirement, and his children under the age of sixteen years, if any, shall be paid out of the pension fund such sum of money as may be determined by said rules and regulations. If there be no widow, or if such widow die while unmarried, the amount of her said benefit may be added to the said benefits for such children. If such member be discharged from the department by reason of age, or if he be found permanently disabled by reason of service in such department, his payment shall be made out of the pension fund. If any retired or pensioned member reenters the service of the department his payments shall abate during such service.