Background: During the COVID-19 pandemic there has been a rise in common mental health problems compared to pre-pandemic levels, especially in young people. Understanding the factors that place young people at risk is critical to guide the response to increased mental health problems. Here we examine whether age-related differences in emotion regulatory processes account for the higher levels of mental health problems reported by young people during the pandemic.Method: Participants (N=2367; 89.95% female, 11-100 years) from Australia, UK, and USA were surveyed thrice at three-month intervals during the first year of the pandemic (May 2020–April 2021). Participants completed measures of emotion regulatory processes (adaptive and maladaptive strategy use and mental flexibility), negative and positive affect, and mental health problems. Results: Younger age was associated with lower positive (p<0.001) and greater negative (p<0.001) affect across the first year of the pandemic. Maladaptive emotion regulation partially accounted for age-related variance in affect (positive affect: p=0.004; negative affect: p<0.001); whereby younger age was associated with greater maladaptive emotion regulation which, in turn, was associated with poorer affect. Maladaptive emotion regulation and its effect on changes in affect, in turn, partially accounted for age-related variance in mental health problems (positive affect: p=0.003; negative affect: p=0.013). Conclusion: Our findings add to the growing literature demonstrating adolescent vulnerability during the COVID-19 pandemic, and suggest that maladaptive emotion regulation may be a promising target for intervention. Targeting such processes, especially in young people, may lead to improvements in affect, and in turn, mental health.