Looks like what might be refered to is the Prouty amendment to a Social Security act of 1966. To qualify for Prouty benefits you seem to have to have turned 72 by 1972 if male and by 1970 if female. I couldn't get any firm information on this but I googled "prouty" "prouty benefits" and "prouty-amendment" and got some stuff. Cf. http://www.ssa.gov/policy/reference/glossary/term/S/specialage72benefit.htm