S.D. Codified Laws § 3-12C-1116

A foundation member's spouse who would be eligible to receive a surviving spouse benefit at age sixty-five may elect to start the benefit prior to age sixty-five but no earlier than the date on which the surviving spouse attains the age of fifty-five. The early surviving spouse benefit, payable for the life of the surviving spouse, is the surviving spouse benefit reduced by five percent for each full year and prorated for each additional full month between the date the early surviving spouse benefit commences and the date the surviving spouse attains the age of sixty-five.