Artificial intelligence is rapidly reshaping materials science by accelerating property prediction, synthesis planning, and materials design. Yet most AI models for materials are developed and validated under implicit stationary assumptions, while real deployments unfold in time-varying environments where materials, sensors, and processes evolve. This review synthesizes what is currently known about temporal generalization in materials AI—the capacity of models to remain reliable as data distributions and underlying mechanisms change. We distinguish two dominant degradation pathways: drift, in which input statistics or input–output relationships shift over time, and model aging, in which learned representations become obsolete as systems evolve. Drawing on evidence across biosensing and wearables, electrochemical energy storage, polymer synthesis, automated laboratories, and industrial manufacturing, we summarize how temporal failures arise, how they are detected, and why they often remain silent until performance drops become consequential. We then evaluate mitigation strategies—including domain adaptation, incremental and continual learning, active data acquisition, uncertainty-aware prediction, and human–AI feedback loops—highlighting where they succeed, where they break down, and the constraints that limit their scalability in real-world settings. Finally, we identify key gaps: limited longitudinal datasets, weak standardization of temporal evaluation protocols, underexplored multimodal temporal fusion, and insufficient emphasis on prevention rather than detection. We conclude with a forward agenda for resilient materials AI built around lifecycle monitoring, benchmarkable temporal stress tests, and hybrid frameworks that integrate mechanistic knowledge with adaptive learning to sustain reliability over time.