According to the demographic classification scheme of the aging population, the "old-old" refers to people who are answer

85+--the "old-old" refer to those over the age of 85 and are typically facing issues of decreasing mobility and increasing illness. 

With increasing life expectancy and changes in lifestyle the elderly population look radically different from the age of 65 (when the elderly category begins) to the point of death. The elderly period is now broken into 3 categories: old (65-74), middle-old (75-84), and old-old (85+).