The first aim of this study was to obtain a more detailed picture of the effect of masker bandwidth (20 Hz to 1000 Hz bandwidth) on the masking level difference (MLD) for a 500-Hz signal as a function of listener age. The results of the pure-tone signal experiment showed that the MLDs of older children differed from adults only for the narrowest masker bandwidth. In contrast, children younger than about 7 years of age tended to have smaller MLDs than adults at all but the widest masker bandwidths. These results suggest that the younger the listener, the wider the noise bandwidth must be for MLDs of adult magnitude to be observed. One interpretation of this effect is that younger listeners require relatively great spectral dissimilarity (and, therefore perceptual dissimilarity) between the signal and masker in order to obtain MLDs of adult magnitude. The second aim of this study was to test this possibility by determining the MLD for noise signals in cases where the signal and masker bandwidths were the same. The results of this experiment showed that the MLDs of children were as large as those of adults when the signal/masker bandwidth was 320 Hz, but were smaller than those of adults when the signal/masker bandwidth was 20 Hz. This indicates that the factor limiting the MLD for narrowband noise in children is related more to the masker bandwidth than to the perceptual similarity between the signal and the masker.