Research on emotion regulation (ER) has increasingly recognized that people use multiple strategies simultaneously, often referred to as ER repertoire. Prior research found that ER repertoire is associated with psychopathology, but results have been mixed. Indeed, research from recent years suggests that it is the quality of ERs, more than their quantity, that needs to be considered. Based on the combination of the literatures on ER repertoire, polyregulation, and ER flexibility, we propose a novel metric: the ratio of using putatively maladaptive (vs. all) ER strategies. Using this metric, we examine (1) maladaptive ER ratio changes during the transition to adolescence, a developmental period in which the prevalence of depression sharply increases, and (2) whether changes in maladaptive ER ratio are associated with depressive symptoms. One-hundred and thirty-nine youths (baseline age: 8–15) reported ER strategies and depression daily for 21 days. One year later, 115 completed another 28-day daily-diary (Nassessments = 5631). Our results show that almost all youth use at least some maladaptive ERs. Importantly, maladaptive ER ratio decreases over a year of adolescence for most youths. Conversely, an increased maladaptive ER ratio predicted depression increases on the daily and on the yearly level. These results shed light on typical and atypical development of ER flexibility and emphasize the need to consider the balance between ERs in relation to psychopathology.