Prior research has shown that school-aged children's metaphor comprehension becomes adult-like progressively. This has given rise to claims that the development of metaphor comprehension is due to children's evolving abilities with respect to Theory of Mind (ToM) or to formal language. The present work investigates the extent to which children's growing sophistication with metaphor is attributable to each of these. Experiment 1 validates a newly constructed tablet task -with two groups of children whose mean ages were approximately 7 and 10 (N=89) -in which participants a) listen to vignettes that conclude with either a metaphoric or a synonymic (control) reference and then; b) choose pictures (while latencies are recorded) that indicate whether the children understand the reference as intended. The outcomes from Experiment 1 confirm prior results: Accurate responding in the wake of a metaphoric reference increases with age; meanwhile, correct metaphoric responses take longer than synonymic ones. Experiment 2 tests a more expansive range of 6-to 11-year-olds (N=248) and a wider array of tasks, including two clinical tasks measuring ToM and formal language skills which we use as cognitive predictors of metaphor accuracy and response times. Results show that ToM is a reliable predictor of successful performance on the metaphor task among younger children, before attenuating with age; in contrast, formal language is a predictor of metaphor comprehension that strengthens with age and is maximal in older children. This work underlines the importance of considering developmental perspectives when investigating the cognitive bases of metaphor skills.