It is well known that walking while talking may be challenging, but does merely keeping a future intention in mind (i.e., prospective memory, PM), even without execution, impose cognitive costs on postural stability? Reaction times and accuracy in a primary cognitive activity used so far may not fully capture these hidden costs, whereas center of pressure (COP) fluctuations offer a promising task-external indicator of cognitive load due to shared cognitive-postural resources. Fifty-three younger (18–30 years) and 47 older adults (60–86 years) performed an arrow task while standing on a force platform. Cognitive load was manipulated across two task-load conditions (low vs. high) and three PM conditions: (1) no PM instruction, (2) an instructed but unexecuted PM task (intended-PM), and (3) an instructed and executed PM task (executed-PM). The Cognition-Balance-Cost-Index (CBCI), integrating reaction times and COP, quantified cognitive-postural resource allocation. Results showed that younger adults’ CBCI remained stable, with only a moderate increase in executed-PM under high load. In contrast, older adults exhibited significantly higher CBCI scores across all conditions, peaking in the executed-PM condition. Notably, intended-PM under high load reduced CBCI in older adults, suggesting compensatory resource allocation, and highlighting postural stability as a valuable, non-invasive indicator of cognitive load.