In the application of Code Sections 47-2-121, 47-2-123, 47-2-124, and subsection (d) of Code Section 47-2-120 to allowances, the monthly benefit of the member, whether dead or in life, or his beneficiary shall be computed as if the member attained age 60 or 65, whichever is applicable, and upon reaching that age applied for a retirement allowance. The monthly benefit so computed shall be payable for the life of the member or pursuant to the terms of the option, if any, selected by the member under Code Section 47-2-121.