Leptin (Lep), a peptide hormone, plays an important role in the regulation of energy metabolism, appetite, and body weight. Despite the well-established dependence of Lep levels on adipose tissue mass, the determinants of circulating leptin concentrations in individuals of different sexes and ages remain insufficiently understood. The aim of this study was to identify the correlates and determinants of plasma Lep concentration in men and women, develop mathematical models for predicting its level, and assess the relationship between leptinemia and the rate of aging. Data from 131 women and 80 men aged 20 to 89 years were analyzed. Participants were divided into two age groups: younger than 60 years and 60 years or older. The study included anthropometric measurements, determination of plasma Lep and insulin concentrations, assessment of serum glucose and lipid levels, heart rate variability (HRV) analysis, and calculation of metabolic age and metabolic aging rate. Plasma Lep concentration was found to be 2.5-fold higher in women than in men. In women, Lep levels were more closely associated with total body fat content, whereas in men they were more strongly related to visceral obesity and insulin resistance. In women younger than 60 years, Lep concentra- tion showed a positive correlation with age; however, this relationship disappeared in the older age group. In men, Lep levels were not associated with age. Among men younger than 60 years, strong correlations were observed between plasma Lep concentration and HRV indices reflecting the activity of both the sympathetic and parasympathetic divisions of the au- tonomic nervous system. In men, higher plasma Lep concentra- tions were also associated with a higher rate of metabolic aging. These findings indicate that the correlates and determinants of circulating leptin concentrations are sex- and age-dependent. These factors should be taken into account when considering Lep as a biomarker of aging and metabolic risk.