S.D. Codified Laws § 3-12C-1216

A generational member's spouse who is eligible to receive a surviving spouse benefit at age sixty-seven may elect to start the benefit in the ten years preceding the spouse attaining the age of sixty-seven. 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-seven.