Objectives: The relationship between cognitive decline and life satisfaction in older adults remains unclear. This study aimed to examine whether older adults with normal cognition at baseline, who developed cognitive decline (i.e., mild cognitive impairment or dementia) over a five-year period, experience lower life satisfaction compared to those who remained cognitively intact.

Methods: The present longitudinal study was conducted in the Arakawa Ward, Tokyo, Japan. Among the initial 1,099 community-dwelling older adults who were between 65 to 84 years old in 2016, we analyzed data from 628 participants who were cognitively intact at baseline and remained in the study at follow-up (2022–2023) with complete data. The effect of developing cognitive decline on life satisfaction, measured by the Satisfaction with Life Scale (SWLS), was examined using a linear mixed model. In addition to developing cognitive decline, fixed effects included time, sex, age, education, depression (indicated by the Geriatric Depression Scale scores above 5), living status (living alone), and frequency of social interactions. The intercept and participants were treated as random effects.

Results: At follow-up, 87 participants were diagnosed with mild cognitive impairment and 7 with dementia. Overall, the mean SWLS score experienced a slight but statistically significant decrease over the five-year period (18.14 ± 5.27 at baseline and 17.75 ± 5.27 at follow-up, p = 0.02 as determined by a paired t-test). The linear mixed model analysis revealed that developing cognitive decline was not significantly associated with life satisfaction (p = 0.93). Notably, living alone and being in a depressive state were significantly associated with higher SWLS scores (p = 0.003 and p < 0.001, respectively).

Conclusions: Life satisfaction did not significantly decline in older adults who developed cognitive decline compared to those who remained cognitively intact. The unexpected findings that individuals living alone and those experiencing depressive states reported higher life satisfaction may indicate potential adaptive mechanisms, whereby these individuals might have developed strategies to find joy in everyday life. Further research is warranted to replicate these results and to explore the underlying factors or potential resilience mechanisms contributing to these findings.