Till date, research on aging in HCI has largely adopted human-centered approaches, such as user-centered and participatory design. However, recent research is beginning to question this “humanistic” focus in aging. Through this dissertation, I provide a case of adopting posthumanist entanglement perspective to understand the ‘more-than-human’ aspects of aging. Posthumanist entanglement perspectives [71]—previously adopted by HCI researchers in different contexts varying from creative design to technical areas such as machine learning and neural networks—attunes to the agency of nonhuman world in addition to the human world to account for how humans and their socio-material worlds are entangled. In Study 1, I investigate older adults’ ontological perceptions with respect to a popular emerging technology to examine the phenomena of “ontological uncertainty” (here, ontologies refer to how things exist and what categories they belong to). Although some researchers adopting entanglement perspectives in HCI argue that ontological uncertainty is posed by emerging technologies such as AI, IoT [71], we lack an understanding of when this uncertainty emerges, and why this matters. Here, the first study of my dissertation focuses on older adults’ use of emerging AI-based voice assistants, and contributes by providing an empirical understanding of the different factors that contribute to ontological uncertainty (e.g., location in house, time, user’s desire for companionship), and provides recommendations for designing voice technologies with ontological categorization in mind. In the next two threads of my work, I attune to the agency of nonhuman entities and how they shape reality associated with older adults’ use of emerging technology (Study 2), and when older individuals engage in designing emerging technology (Study 3). My analysis from Study 2 reveals how nonhuman actors such as materials and norms play a role in shaping older adults’ preference and use of voice technologies. My findings also reveal the salient ways in which voice assistants play an active role in mediating relations between older adults and their larger social world. These mediations are shaping our social practices around what it means to live alone, to give company, or to give and receive care. Finally, my analysis from Study 3— which adopts a posthumanist perspective to understand older adults’ engagement in design workshops— reveals the nuanced ways in which designs materials (both expected and unexpected) act in relation to older adults: from facilitating creative brainstorming, to limiting creative brainstorming, to leading to clashing of ideas, and contributing to non-participation in the design activity. My findings also reveal how older adults went beyond focusing on just the technology idea to account for the physical objects or the environment associated with both technology use and non-use, thus bringing to attention that technology cannot be seen, used, or designed in isolation, and exists within specific configurations of actor-networks. Overall, my thesis contributes by providing insights on the new directions that HCI researchers working on aging can take in terms of: a) taking into account the ways in which the nonhuman entities act and hold them accountable for undesired realities, b) designing emerging technologies that support meaningful relationships between older adults and their world, and c) move beyond designing technology in isolation to instead purposefully situate older adults in designing meaningful configurations of human and nonhuman entities (including technology).