This study develops a traceable design-translation framework linking intergenerational social-sustainability concerns, mutual-learning mechanisms, user needs, and product-design elements, and illustrates its application through a home-gardening product concept for grandparent–grandchild dyads. Equitable participation, reciprocal knowledge contribution, social connectedness, and continuity of engagement are treated as design concerns rather than measured sustainability outcomes. Mutual-learning mechanisms are organized across three stages of collaborative gardening: preparation and initiation, collaborative engagement, and reflection and continuation. Behavioral observations and semi-structured interviews with eight dyads informed a KJ-based synthesis of user needs, which were prioritized using the Analytic Hierarchy Process (AHP) and translated through the Function–Behavior–Structure (FBS) model into seven functions and eight core structural elements. A descriptive concept evaluation with 30 dyads compared the proposed concept with conventional home gardening under six design-derived criteria. The proposed concept received higher mean ratings across all six criteria, with composite indices of 0.857 and 0.539, respectively. These results reflect concept-level responses rather than evidence of actual behavioral change or long-term social-sustainability outcomes. The main contribution is the documented design-translation framework, while the gardening concept serves as an illustrative application.