Many older adults are interested in smartphones but encounter difficulties in self-instruction and need support, especially text input. Voice input is a useful option for text input, but also presents some difficulties for older adults. In this paper, we propose a tutoring system for voice input that detects input stumbles using a statistical approach and provides instructions to overcome them. We construct the tutoring system based on the data from a user study with novice older adults. In an evaluation experiment, the number of input stumble and the sentence completion time of the participants using the tutoring system were significantly smaller than those without it. The results showed that the tutoring system resulted in the improvement of the efficiency of voice input for novice older adults.