While difficulties with emotion regulation (ER) are consistently linked to poor mental health in adulthood, findings in adolescence have been more mixed. Cognitive ER strategies, which involve the ability to manage emotions through mental processes, may be particularly important during different stages of development due to age-specific adjustments. We conducted two exploratory and cross-sectional studies to examine the relationships between cognitive ER strategies and mental health (i.e., depressive, anxiety, and insomnia symptoms) in two samples: 431 young adults (M = 20.66 ± 2.21; 70% women and 30% men) and 271 adolescents (M = 14.80 ± 0.0.59; 44.6% girls and 55.4% boys). The participants completed a group of questionnaires, including the Cognitive Emotion Regulation Questionnaire, the Insomnia Severity Index, the Beck Depression Inventory-II, the State-Trait Anxiety Inventory, and the Youth Self Report. We employed hierarchical multiple regressions to assess the unique contribution of cognitive ER strategies to mental health outcomes. Maladaptive strategies (such as rumination and catastrophizing) were consistently associated with impaired mental health in both samples, while adaptive strategies (such as positive refocusing and positive reappraisal) were only associated with better mental health in young adults. These findings support the importance of cognitive ER strategies as potential risk factors for psychopathology and suggest that interventions aimed at improving emotion regulation may be beneficial. The age-specific differences in the relationship between cognitive ER strategies and mental health may reflect the refinement of emotion regulation abilities across the lifespan.