The design of reasonable reward mechanisms in exercise games aims to increase people’s intrinsic motivation to participate in the games for training so that they can persist for a long time and achieve exercise results. This study developed two game modes based on a prototype of an exercises game for a Kinect interactive device. Two reward mechanisms, in-game rewards and extrinsic rewards, were included in one of the game modes respectively. Twelve older subjects and twelve younger subjects were recruited to test the game. They rated the game using an intrinsic motivation scale. The results showed that young people who played the in-game reward mode scored higher on the motivation scale. In contrast to young people, older people had higher motivation levels when playing the extrinsic reward mode. The results highlight the variability in the effects of in-game and extrinsic rewards on intrinsic motivation across age groups and can help people better design reward mechanisms when developing exercise games.