Verbal and spatial span, articulation and tapping rate, and verbal and spatial speed-of-search tasks were administered to sixty 6- to 12-year-olds. A variance-partitioning procedure was then used to identify age-related and age-invariant components of variance in span. Outcomes were very similar for verbal and spatial span, in particular, (i) most of the age-related variance was shared by the speed-of-search and rate predictors, (ii) articulation rate provided an age-independent contribution, (iii) changing-state versions of predictor tasks did not account for variance over steady-state versions, and (iv) predictors of the same modality as the span task did not outperform predictors of the other modality. We conclude that verbal and spatial short-term memory appear to rely on similar processes when serial recall is required and that developments in span are closely tied to increases in processing speed.