Parkinson's disease (PD) and dementia are major age-related neurodegenerative disorders imposing a growing public health burden in aging societies such as Korea, yet long-term national evidence on incidence and sociodemographic disparities remains limited. We conducted a nationwide population-based study using the National Health Insurance Service database from 2003 to 2023. Age-standardized and age-specific incidence rates were calculated, and temporal changes were assessed using joinpoint segmented regression with policy-relevant breakpoints. PD incidence declined modestly but significantly from 16.6 in 2003 to 11.4 per 100,000 person-years in 2023 (AAPC: -2.1, 95% CI: -2.7, -1.5). In contrast, dementia incidence increased sharply from 41.9 to 171.6 per 100,000 person-years between 2003 and 2012, followed by a sustained decline after 2018 to 112.7 in 2023; however, the overall trend showed a significant increase (AAPC: 5.2, 95% CI: 3.7, 6.5). Incidence of both conditions peaked among individuals in their 70s and 80s. Dementia disproportionately affected women, rural residents, and medical aid recipients. Age-related declines in disease-free survival were greater for dementia, with the lowest survival observed among medical aid recipients and rural residents. These findings indicate an uneven population-level burden of neurodegenerative diseases and the need for equitable health system strategies amid ongoing population aging.