Background: Several theorists maintain that exact number abilities rely on language-relevant processes whereas approximate number calls on visuo-spatial skills. We chose two genetic disorders, Williams syndrome and Down's syndrome, which differ in their relative abilities in verbal versus spatial skills, to examine this hypothesis. Five experiments assessed number skills in these two genetic syndromes and in their mental age (MA) and chronological age (CA) matched controls. Methods: Experiment 1 used a preferential looking paradigm with infants and toddlers to measure sensitivity to changes in numerosity. Experiment 2 measured reaction times in older children and adults in a numerosity comparison task with dots in a random pattern. Experiment 3 comprised a number battery that measured various forms of counting and simple arithmetic. Results: The WS infants displayed a level of performance equal to that of their CA-controls, whereas the DS infants failed to reach even the level of their MA-controls. By contrast, the older DS children and adults outstripped the older WS group in their numerosity abilities, with different patterns of errors in the two clinical groups. Conclusions: Differences in the infant and adult number phenotypes between these two genetic disorders are discussed with reference to the processing styles used by each group and how these might impact on their developmental trajectories. Theoretically, we highlight our contention that one cannot infer the infant starting state from the adult end state. Rather, the development process itself must be taken into account.