The association of fasting glucose with the risk of death according to sex and age remains unclear, and insufficient information is available on sex- and age-specific glucose concentrations within ethnic groups. This study analyzed a sample of 12,455,361 Korean adults who participated in health examinations during 2001-2004, and were followed up until 2013. Men had 3.0 mg/dL (0.167 mmol/L) higher mean glucose concentrations than women (94.7 vs. 91.7 mg/dL), although women over 73 years had higher levels. For glucose levels of 100-199 mg/dL, each 18 mg/dL (1 mmol/L) increase in fasting glucose increased mortality by 13% (HR = 1.13, [95% CI 1.12 to 1.13], p < 0.001). In individuals with fasting glucose levels of 100-125 mg/dL, each 18 mg/dL increase in fasting glucose was associated with a 30% increase in the risk for mortality (1.30, [1.18 to 1.43]) in those aged 18-34 years, a 32% increase (1.32, [1.26 to 1.39]) in those aged 35-44 years, and a 10% increase (1.10, [1.02 to 1.19]) in those aged 75-99 years. The fasting glucose levels associated with the lowest mortality were 80-94 mg/dL regardless of sex and age. Prediabetes (100-125 mg/dL) was associated with higher mortality. The associations of hyperglycemia with mortality were stronger at younger ages.