Healthy aging beyond the age of 65 is characterized by a general decrease in cognitive control over actions: old adults have more difficulty than young adults in stopping overt responses. Responsible for this cognitive decrement is the continuous decline of striatal and extrastriatal dopamine (DA). The resource-modulation hypothesis assumes that genetic variability is more likely to result in performance differences when brain resources move away from close-to-optimal levels, as in aging. To test this hypothesis we investigated, first, whether individual differences in the C957T polymorphism at DRD2 gene (rs6277) contribute to individual differences in the proficiency to inhibit behavioral responses in a stop-signal task. Second, we assessed whether this genetic effect is magnified in older adults, due to the considerable decline in dopamine function. Our findings show that individuals carrying genotype associated with higher density of extrastriatal D2 receptors (C957T CC) were more efficient in inhibiting unwanted action tendencies, but not in term of response execution. This effect was stronger in older than in younger adults. Our findings support the idea that aging-related decline in dopamine availability alters the balance between genotypes and cognitive functions.