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As global life expectancy rises, understanding predictors of survival in extreme old age is crucial. Body mass index (BMI) is a widely used proxy for adiposity and nutritional status. In adults, a BMI between 18.5 and 24.9 kg/m<sup>2</sup> is considered healthy and associated with better life expectancy; yet in nonagenarians and centenarians it remains unclear whether this BMI range is ideal due to age-related physiological changes. We examined BMI and mortality associations in 780 adults aged ≥90 years (mean age = 99.25 ± 3.3 years) from the CEPH Aging Cohort. Over six years of follow-up, 88.3% of participants died. Cox models revealed that each 5 kg/m<sup>2</sup> increase in BMI was associated with a 16% reduction in mortality risk (HR = 0.97 per 1 kg/m<sup>2</sup> increase; 95% CI: 0.95-0.99; p = 0.001), with age as the time scale and adjusted for sex, cardiovascular disease, vulnerability indicator, neurocognitive status, and smoking status. Age-specific analyses suggested that underweight status was associated with higher mortality in the oldest age groups. The accelerated failure-time model confirmed 23% longer survival with higher BMI (time ratio 1.23; p < 0.0001). Normal and overweight individuals outlived those underweight, with no survival difference between the former groups. These findings indicate that BMI-mortality associations at very advanced ages differ from those observed in younger adults, with higher BMI among nonagenarians and centenarians serving as a marker of longer subsequent survival. Age-specific weight targets may improve longevity and resilience in this rapidly growing demographic.