SoK: Reviewing Two Decades of Security, Privacy, Accessibility, and Usability Studies on Internet of Things for Older Adults

The Internet of Things (IoT) has the potential to enhance older adults' independence and quality of life, but it also exposes them to security, privacy, accessibility, and usability (SPAU) risks. We conducted a systematic review of 44 peer-reviewed studies published between 2004 and 2024 using a five-phase screening pipeline. From each study, we extracted data on study design, IoT type, SPAU measures, and identified research gaps. We introduce the SPAU-IoT Framework, which comprises 27 criteria across four dimensions: security (e.g., resilience to cyber threats, secure authentication, encrypted communication, secure-by-default settings, and guardianship features), privacy (e.g., data minimization, explicit consent, and privacy-preserving analytics), accessibility (e.g., compliance with ADA/WCAG standards and assistive-technology compatibility), and usability (e.g., guided interaction, integrated assistance, and progressive learning). Applying this framework revealed that more than 70% of studies implemented authentication and encryption mechanisms, whereas fewer than 50% addressed accessibility or usability concerns. We further developed a threat model that maps IoT assets, networks, and backend servers to exploit vectors such as phishing, caregiver exploitation, and weak-password attacks, explicitly accounting for age-related vulnerabilities including cognitive decline and sensory impairment. Our results expose a systemic lack of integrated SPAU approaches in existing IoT research and translate these gaps into actionable, standards-aligned design guidelines for IoT systems designed for older adults.