Sleep spindles are a physiological marker of off-line memory consolidation. In young adults, sleep spindles are preferentially responsive to encoded information that is tagged as having future relevance. Older adults, on the other hand, show reduced capacity for future simulation and alterations in sleep physiology. Healthy young adults ( n = 38) and older adults ( n = 28) completed an adaptation night, followed by two in-laboratory polysomnography nights, in which they mentally simulated future events or remembered past events, recorded via written descriptions. We quantified the degree of future/past thinking using linguistic analysis of time orientation. In young adults, greater future thinking was linked to greater spindle density, even when controlling for gender, age, and word count ( r p = .370, p = .028). The opposite was true for older adults, such that greater future thinking was associated with reduced spindle density ( r p = −.431, p = .031). These patterns were selective to future thinking (not observed for past thinking). The collective findings implicate an impaired interaction between future relevance tagging and sleep physiology as a mechanism by which aging compromises sleep-dependent cognitive processing.