Act as a seasoned sociologist with expertise in futuristic societal structures. Design a detailed framework for a society where individuals can choose when and how they age. Consider the following aspects: [economic systems] that support non-linear aging, [cultural norms] around life milestones, and [governance models] to manage population dynamics. Address potential challenges such as overpopulation, generational gaps, and psychological impacts of eternal youth. Provide innovative solutions to ensure sustainability and social harmony. Your response should include specific policies, technologies, and cultural practices that would make this society viable.