This paper presents key aspects and trade-offs that designers and Human-Computer Interaction practitioners might encounter when designing multimodal interaction for older adults. The paper gathers literature on multimodal interaction and assistive technology, and describes a set of design challenges specific for older users. Building on these main design challenges, four trade-offs in the design of multimodal technology for this target group are presented and discussed. To highlight the relevance of the trade-offs in the design process of multimodal technology for older adults, two of the four reported trade-offs are illustrated with two user studies that explored mid-air and speech-based interaction with a tablet device. The first study investigates the design trade-offs related to redundant multimodal commands in older, middle-aged and younger adults, whereas the second one investigates the design choices related to the definition of a set of mid-air one-hand gestures and voice input commands. Further reflections highlight the design trade-offs that such considerations bring in the process, presenting an overview of the design choices involved and of their potential consequences.