Objective : This study aimed to precisely analyze the changing patterns of Basic Activities of Daily Living (BADL), Instrumental Activities of Daily Living (IADL), and depression among Korean older adults by stratifying age into 5-year intervals. Furthermore, it examined the impact of depression on functional disability, specifically comparing its relative risk for BADL versus IADL impairment. Methods : A secondary analysis was conducted using data from the 9th Korean Longitudinal Study of Ageing(KLOSA, 2022), involving 4,489 participants aged 65 years and older. Data were analyzed using descriptive statistics, independent t-tests, one-way ANOVA, chi-square tests, and calculate Odds Ratios (OR). Results : First, significant functional decline was observed starting at age 75. Notably, BADL and IADL scores, along with the prevalence of depression, showed a precipitous increase after age 80. Second, item-specific analysis revealed that ‘bathing/showering' (BADL) and ‘locomotion using transportation' (IADL) were the functions most vulnerable to aging. Third, older adults with depression exhibited a 5.32-fold increased risk of IADL disability and a significantly higher 8.36-fold risk of BADL disability— which is essential for survival—compared to their non-depressed counterparts. Conclusion : This study confirms that functional decline in older adults begins around the age of 75 and accelerates rapidly after age 80. Furthermore, the findings demonstrate that depression is a more detrimental risk factor for physical self-maintenance (BADL) than for social functioning (IADL). Consequently, age 75 should be established as a practical functional threshold to differentiate between prevention and care programs, and age 80 should be considered a benchmark for intensive care. Integrated psychophysical intervention strategies are essential to disrupt the vicious cycle between depression and physical deterioration.