This pilot study utilizes a voice assistant system (VAS) as a noninvasive method for the early diagnosis of cognitive decline. We collected voice command data from 35 older adults (15 of whom interacted with the VAS daily) over 18 months. To address the challenges of analyzing short, irregular, and noisy voice commands, we propose the Cog-TiPRO framework. Cog-TiPRO combines linguistic feature extraction through LLM-based repetition prompt refinement, HuBERT-based acoustic feature extraction, and Transformer-based temporal modeling. Using iTransformer, we achieved 73.80% accuracy and 72.67% F1-score for Mild Cognitive Impairment (MCI) detection, a 27.13% improvement over existing methods. The LLM approach identified linguistic features that characterize the daily command usage patterns of individuals experiencing cognitive decline.