In recent years, the elderly population has increased, and most of them tend to live alone. This means that the elderly need to communicate with their younger colleagues, friends, and family via smartphones because these devices provide several channels of communication, such as calling, chatting, and video conferencing. However, the main problem for the elderly is their physical condition which inhibits them from using smartphones. Thus, the objective of this paper is to find out what are the UI components that degrade their ability for using smartphones. This can be achieved by comparing the capabilities of using smartphones between younger and elderly users and finding out the UI components what cause their capabilities to be different. From our assumption that physical conditions limit the capability of the elderly, we focused our research on their visual capabilities by conducting the experiments on younger users and elderly users, who were requested to perform specific tasks on Line, which is one of the most popular communication applications among Thais. The experimental results reveal that most of the elderly people take longer time to complete their tasks and make more mistakes than younger users. It was found that most of them have problems with color, font style, size, and brightness of UI. As a consequence, it is suggested that a more appropriate UI for smartphone applications, especially communication applications, should be designed to solve such problems in order to increase the elderly's capabilities of using smartphones.