The emerging United Nations process toward a legally binding instrument on the human rights of older persons is not only a legal milestone; it is an accountability test for geriatric psychiatry, brain health, dementia care, mental health services, long-term care, community care, palliative care, digital systems, and implementation science. The legal protection gap has been documented through the Madrid International Plan of Action on Ageing, the Open-ended Working Group on Ageing, OHCHR analyses, Independent Expert reports, and legal scholarship. As the Human Rights Council moves this work into an Intergovernmental Working Group to elaborate a legally binding instrument, geriatric psychiatry and allied fields have a responsibility to contribute clinical, ethical, scientific, and implementation expertise. This essay argues that the future Convention should be understood not only as a legal text but as an opportunity to make dignity, agency, legal capacity, supported decision-making, brain health, mental health, care, support, participation, and freedom from discrimination real in everyday systems. Ageism causes measurable health harms; mentalism and ableism compound exclusion; dementia and cognitive impairment can trigger diagnosis-based loss of voice and agency; grief, loneliness, coercion, and institutionalization reveal failures of supportive care; and artificial intelligence can deepen or mitigate ageism, digital exclusion, and epistemic injustice. Strengths-based approaches emphasizing wisdom, resilience, community capacity, and learning health systems offer a way forward. Building on the WPA-SOAP White Paper, Leiden Summary Statement, and WHO/OHCHR tools, this essay offers an implementation-focused bridge from principle to practice and from aspiration to accountability.