This paper presents the design and implementation of a standing/sitting assist robot, helping elderly people support their balances in daily routines. Specifically, according to their standing and/or sitting behaviors, the height of the armrest in the assist robot is designed to be controlled. For this purpose, the location tracking model for the postural status of user's lower limbs when standing or sitting is proposed, to enable the robot to estimate and predict the knee locations. Based on the model, the proposed scheme can control the movement of the armrest corresponding to the user's behaviors. Details on the robot realization and its control scheme are explained, and its effectiveness is verified through experiments.