Purpose Replacing irrational or unhelpful thoughts with more rationale and helpful ones is a core skill in Cognitive Behavioural Therapy, but there is limited research examining the neuropsychological mechanisms underpinning this process. This study examined age-differences in cognitive restructuring skill acquisition between children, younger adults and older adults; whether individual differences in cognitive flexibility influences the ability to learn cognitive restructuring across the lifespan; and whether cognitive flexibility explains age-differences in cognitive restructuring ability. Method Participants were 114 individuals with anxiety disorders, including 35 children (aged 7–12, M = 9.14, SD = 1.44), 32 younger adults (aged 18–53, M = 23, SD = 7.84) and 47 older adults (aged 61–78, M = 66.81, SD = 4.43). Participants completed neuropsychological measures of cognitive flexibility and learned cognitive restructuring, which was coded for quality and efficacy. Results More than half the participants of all ages showed good quality cognitive restructuring skill acquisition with only brief instruction. Older adults showed comparable cognitive restructuring skills to children, with slightly better skills among younger adults. However, after accounting for individual differences in cognitive flexibility, there were no age-differences in cognitive restructuring quality. Greater perseveration was associated with poorer cognitive restructuring skill acquisition in younger and older adults, and mediated age-differences in cognitive restructuring skill acquisition. Conclusions Among younger and older adults, individual differences in perseveration, rather than chronological age, underpins the ability to learn cognitive restructuring. There is little evidence that cognitive flexibility plays a role in cognitive restructuring skill acquisition among children.