Intergenerational programs (IGPs) can foster interaction and collaboration between different age groups. However, most IGPs are led by younger people with limited involvement from older adults in activity design and planning. This study aims to clarify the benefits and challenges of involving older adults in IGP design and planning. We organized an IGP involving seven older and six younger participants in a community in Shanghai, and older participants actively contributed to planning, organizing, and execution throughout the IGP. We distributed questionnaires before and after the IGP to quantify its benefits for mental health and conducted in-depth interviews to identify specific benefits and challenges for both age groups. The results supported those older adults who felt less loneliness and perceived higher self-efficacy, self-value, and well-being by participating in IGP design and planning. Younger people reported lower ambivalent ageism. Both age groups perceived stronger senses of meaning in life. In addition, we identified challenges and provided recommendations for involving older adults in IGP planning.