In this study, we observed the behavior of younger adults (20-29 years old) and middle-aged adults (46-59 years old) interacting with complicated electronic devices. Two recently released multi-functional multimedia devices, namely PMPs (portable multimedia players) and MP3 players were used in the observations. We examined various aspects of interaction behaviors in terms of performance, strategies, error consequences, physical operation methods, and workload. Our analysis of age-related differences included differences in background knowledge as an important independent factor. The results revealed that differences in age meaningfully affected the observed error frequency, the number of interaction steps, the rigidity of exploration, the success of physical operation methods, and subjective perception of temporal demand and performance. In contrast, trial-and-error behavior and frustration levels were influenced by background knowledge rather than age. These novel findings provide important new insights into user interaction characteristics between different age groups and may facilitate the design of age group-appropriate interfaces for complicated electronic devices. (c) 2007 Elsevier Ltd. All rights reserved.