Sensitivity to auditory rhythmic structures in music and language is evident as early as infancy, but performance on beat perception tasks is often well below adult levels and improves gradually with age. While some research has suggested the ability to perceive musical beat develops early, even in infancy, it remains unclear whether adult-like perception of musical beat is present in children. The capacity to sustain an internal sense of the beat is critical for various rhythmic musical behaviors, yet very little is known about the development of this ability. In this study, 223 participants ranging in age from 4 to 23 years from the Las Vegas, Nevada, community completed a musical beat discrimination task, during which they first listened to a strongly metrical musical excerpt and then attempted to sustain their perception of the musical beat while listening to a repeated, beat-ambiguous rhythm for up to 14.4 s. They then indicated whether a drum probe matched or did not match the beat. Results suggested that the ability to identify the matching probe improved throughout middle childhood (8-9 years) and did not reach adult-like levels until adolescence (12-14 years). Furthermore, scores on the beat perception task were positively related to phonological processing, after accounting for age, short-term memory, and music and dance training. This study lends further support to the notion that children's capacity for beat perception is not fully developed until adolescence and suggests we should reconsider assumptions of musical beat mastery by infants and young children.