The aim of the study was to find out the body composition features in persons of the first period of adulthood with different aging rates. Methods. The 2404 patient (607 men and 1797 women) of the first period of adulthood were observed. Body length, body weight, waist and hip circumferences were recorded according to generally accepted methods. The body composition was studied by the bioimpedance measurement. The biological age and the aging rate were calculated using the formulas of A.G. Gorelkin and B.B. Pinkhasov. Results. The majority of men (57.5%) had an accelerated aging rate, the majority of women (51.2%) had a delayed aging. The biological age of 19.1% men and 15.2% women coincided with the calendar age. Body composition differed statistically significantly in group with different aging rates and sex groups. The content of the relative fat mass increased and the relative lean, musculoskeletal, active cell masses decrease with increasing aging rate. The proportion of men and women with accelerated aging in the group with increased fat mass was 92.5% and 98.4% respectively. Excess fat mass increases the chances of premature aging by 5.5 times in men and 1.3 times in women in compared to individuals with optimal fat mass. Conclusion. The aging rate in men was highly correlated with fat, musculoskeletal masses and total body water, in women - with fat mass and total body water. High fat mass contributed to premature aging.