Timely dementia detection determines access to care planning, supportive services, and emerging treatments, yet whether healthcare system has become better at identifying dementia—and whether this varies with the payment arrangement under which beneficiaries receive care—is unknown. We link older adults with probable dementia in Health and Retirement Study to Medicare claims (1998–2020) and estimate the covariate-adjusted share receiving a claims-based dementia diagnosis by year. Among Traditional Medicare (TM) enrollees, fewer than half of all cases—and fewer than one-third of early-stage cases—had ever been diagnosed, showing no significant improvement. Annual detection was higher under fee-for-service TM than under capitated Medicare Advantage (MA) plans. Annual detection rose over time in both plan types, but not among early-stage cases, a pattern more consistent with greater recognition of advanced disease than with upcoding following the 2019 reintroduction of dementia into MA risk adjustment. The results reveal a persistent gap between the policy emphasis on early diagnosis and clinical practice, and suggest that payment structure and provider networks, not only enrollee composition, shape how well dementia is identified.