Purpose <p>To examine caregiver age and mutuality as moderators in the relationship between caregiver strain and quality of life (QoL) among caregivers of offspring with intellectual and developmental disabilities (I/DD).</p> Methods <p>A cross-sectional online survey was conducted with 186 parents of offspring with I/DD. Moderation and three-way interaction analyses were performed using Hayes’ PROCESS macro to test caregiver age, mutuality, and their interaction as moderators of the association between caregiver strain and quality of life.</p> Results <p>The three-way interaction (caregiver strain × mutuality × caregiver age) was statistically significant (<i>p</i> = .01). Higher mutuality buffered the negative impact of caregiver strain on quality of life, but only among younger caregivers.</p> Conclusion <p>Mutuality serves as an important buffer against the detrimental effects of caregiver strain on quality of life for younger caregivers. However, this buffering effect diminishes as caregivers age. Interventions may benefit from an age-tailored approach, such as enhancing mutuality and relational support for younger caregivers while prioritizing tangible, practical support for older caregivers. These strategies could strengthen the design and delivery of caregiver support programs and policies.</p>