Purpose: For colocated targets and maskers, binaural listening typically offers a small but significant advantage over monaural listening. This study investigated how monaural asymmetry and target--masker similarity may limit binaural advantage in adults and children. Method: Ten Mandarin-speaking Chinese adults (aged 22-27 years) and 12 children (aged 7-14 years) with normal hearing participated in the study. Monaural and binaural speech recognition thresholds (SRTs) were adaptively measured for colocated competing speech. The target-masker sex was the same or different. Performance was measured using headphones for three listening conditions: left ear, right ear, and both ears. Binaural advantage was calculated relative to the poorer or better ear. Results: Mean SRTs were significantly lower for adults than children. When the target--masker sex was the same, SRTs were significantly lower with the better ear than with the poorer ear or both ears (p < 0.05). When the target--masker sex was different, SRTs were significantly lower with the better ear or both ears than with the poorer ear (p < 0.05). Children and adults similarly benefitted from target--masker sex differences. Substantial monaural asymmetry was observed, but the effects of asymmetry on binaural advantage were similar between adults and children. Monaural asymmetry was significantly correlated with binaural advantage relative to the poorer ear (p = 0.004), but not to the better ear (p = 0.056). Conclusions Binaural listening may offer little advantage (or even a disadvantage) over monaural listening with the better ear, especially when competing talkers have similar vocal characteristics. Monaural asymmetry appears to limit binaural advantage in listeners with normal hearing, similar to observations in listeners with hearing impairment. While language development may limit perception of competing speech, it does not appear to limit the effects of monaural asymmetry or target--masker sex on binaural advantage.