Sixty normally-hearing listeners, ages 5 to 61 years, participated in a monaural speech understanding task designed to assess the impact of a single-talker speech masker presented to the opposite ear. The speech targets were masked by ipsilateral speech-spectrum noise. Masker level was fixed and target level was varied to estimate psychometric functions. The target/masker ratio that led to 51% correct performance in this task was taken as the baseline threshold. The impact of a modulated speech-spectrum noise, a male talker, or a female talker presented at a fixed level to the contralateral ear was quantified by the change in the baseline threshold and was assumed to reflect informational masking. The modulated-noise masker produced no informational masking across the entire age range. Speech maskers produced as much as 20 dB of informational masking for children aged 5-8 years and only 4 dB for adults. In contrast with previous studies using ipsilateral speech maskers, the male and female contralateral speech maskers produced comparable informational masking. Analyses of the developmental rate of change for informational masking and of the patterns of individual differences suggest that the informational masking produced by contralateral and ipsilateral maskers may be mediated by different mechanisms or processes.