This paper presents the results of a study on noninvasive blood glucose prediction using heart rate variability (HRV) during sleep. Considering age-related autonomic changes that affect conventional HRV analysis, we analyzed multimodal data, including sleep-stage ECGs, HRV features, and clinical measurements, from 43 subjects. We applied a novel technique to normalize HRV features by an age-dependent scaling factor and used Bayesian ridge regression to predict log blood glucose levels. Age-normalized HRV features achieved an R² = 0.161 (MAE = 0.182) for log blood glucose prediction, demonstrating a 25.6% improvement over unnormalized features (R² = 0.132). The most predictive features were age-normalized mean RR interval during REM sleep, age-normalized mean RR interval during deep sleep, and diastolic blood pressure. Further analysis confirmed the importance of age normalization, while sleep-stage features provided additional predictive power.