MobiBox, an innovative health intervention system, is designed to provide proactive and personalized support for the elderly through their own smartphones. Current health applications often fail to adapt to the unique routines and needs of older adults, offering generic, one-size-fits-all reminders. Our system addresses this gap by leveraging a specialized, small-scale Large Language Model (LLM) trained specifically for health intervention. By passively collecting and analyzing multimodal data from the user's smartphone, MobiBox develops a deep understanding of individual behavior patterns, enabling it to deliver timely, context-aware, and truly personalized health advice. This transforms the smartphone into an intelligent, adaptive health companion for promoting active and healthy aging.